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Welllll.....

I know that the other versions are not as good as the KJV, but let's see some of them:

New International Version (©1984)
Now I rejoice in what was suffered for you, and I fill up in my flesh what is still lacking in regard to Christ's afflictions, for the sake of his body, which is the church.
New Living Translation (©2007)
I am glad when I suffer for you in my body, for I am participating in the sufferings of Christ that continue for his body, the church.
New American Standard Bible (©1995)
Now I rejoice in my sufferings for your sake, and in my flesh I do my share on behalf of His body, which is the church, in filling up what is lacking in Christ's afflictions.
International Standard Version (©2008)
Now I am rejoicing while suffering for you as I complete in my flesh whatever remains of the Messiah's sufferings on behalf of his body, which is the church.
GOD'S WORD® Translation (©1995)
I am happy to suffer for you now. In my body I am completing whatever remains of Christ's sufferings. I am doing this on behalf of his body, the church.
King James Bible
Who now rejoice in my sufferings for you, and fill up that which is behind of the afflictions of Christ in my flesh for his body's sake, which is the church:
American King James Version
Who now rejoice in my sufferings for you, and fill up that which is behind of the afflictions of Christ in my flesh for his body's sake, which is the church:
American Standard Version
Now I rejoice in my sufferings for your sake, and fill up on my part that which is lacking of the afflictions of Christ in my flesh for his body's sake, which is the church;
Bible in Basic English
Now I have joy in my pain because of you, and in my flesh I undergo whatever is still needed to make the sorrows of Christ complete, for the salvation of his body, the church;
Douay-Rheims Bible
Who now rejoice in my sufferings for you, and fill up those things that are wanting of the sufferings of Christ, in my flesh, for his body, which is the church:
Darby Bible Translation
Now, I rejoice in sufferings for you, and I fill up that which is behind of the tribulations of Christ in my flesh, for his body, which is the assembly;
English Revised Version
Now I rejoice in my sufferings for your sake, and fill up on my part that which is lacking of the afflictions of Christ in my flesh for his body's sake, which is the church;
Webster's Bible Translation
Who now rejoice in my sufferings for you, and fill up that which is behind of the afflictions of Christ in my flesh for his body's sake, which is the church:
Weymouth New Testament
Now I can find joy amid my sufferings for you, and I fill up in my own person whatever is lacking in Christ's afflictions on behalf of His Body, the Church.
World English Bible
Now I rejoice in my sufferings for your sake, and fill up on my part that which is lacking of the afflictions of Christ in my flesh for his body's sake, which is the assembly;
Young's Literal Translation
I now rejoice in my sufferings for you, and do fill up the things lacking of the tribulations of the Christ in my flesh for his body, which is the assembly,

Looks like the only one that agrees with the KJV is the Douay. Interesting. Hmmmmm...

Let's take a look at what some prominent Protestant commentators had to say about this:

Geneva Study Bible
Who now rejoice in my sufferings for {r} you, and fill up {s} that which is behind of the afflictions of Christ in my flesh for his body's sake, which is the church:
(r) For our profit and benefit.
(s) The afflictions of the Church are said to be Christ's afflictions, by reason of that fellowship and knitting together that the body and the head have with one another. And this is not because there is any more need to have the Church redeemed, but because Christ shows his power in the daily weakness of his own, and that for the comfort of the whole body.
People's New Testament
1:24 Who now rejoice. Rather, I now rejoice.
My sufferings for you. He was a prisoner and a sufferer because he preached the gospel of the Gentiles.
And fill up that which is behind of the afflictions of Christ. Christ endured afflictions for us. We must have a fellowship of his sufferings (1Pe 4:13). Paul also suffers that he might share the afflictions of Christ. Not only did he suffer with Christ, but Christ suffers with his afflicted people. See Ac 9:4. Hence, afflictions suffered for Christ may be called his afflictions. All the suffering required for the sake of the church is behind of the afflictions of Christ.
Wesley's Notes
1:24 Now I rejoice in my sufferings for you, and fill up - That is, whereby I fill up. That which is behind of the sufferings of Christ - That which remains to be suffered by his members. These are termed the sufferings of Christ, Because the suffering of any member is the suffering of the whole; and of the head especially, which supplies strength, spirits, sense, and motion to all. Because they are for his sake, for the testimony of his truth. And these also are necessary for the church; not to reconcile it to God, or satisfy for sin, (for that Christ did perfectly,) but for example to others, perfecting of the saints, and increasing their reward.
Jamieson-Fausset-Brown Bible Commentary
24. Who-The oldest manuscripts omit "who"; then translate, "Now I rejoice." Some very old manuscripts, and the best of the Latin versions, and Vulgate, read as English Version. To enhance the glory of Christ as paramount to all, he mentions his own sufferings for the Church of Christ. "Now" stands in contrast to "I was made," in the past time (Col 1:23).
for you-"on your behalf," that ye may be confirmed in resting solely on Christ (to the exclusion of angel-worship) by the glorification of Christ in my sufferings (Eph 3:1).
fill up that which is behind-literally, "the deficiencies"-all that are lacking of the afflictions of Christ (compare Note, see on [2408]2Co 1:5). Christ is "afflicted in all His people's afflictions" (Isa 63:9). "The Church is His body in which He is, dwells, lives, and therefore also suffers" [Vitringa]. Christ was destined to endure certain afflictions in this figurative body, as well as in His literal; these were "that which is behind of the afflictions of Christ," which Paul "filled up." His own meritorious sufferings in expiation for sin were once for all completely filled up on the Cross. But His Church (His second Self) has her whole measure of afflictions fixed. The more Paul, a member, endured, the less remain for the rest of the Church to endure; the communion of saints thus giving them an interest in his sufferings. It is in reference to the Church's afflictions, which are "Christ's afflictions, that Paul here saith, "I fill up the deficiencies," or "what remain behind of the afflictions of Christ." She is afflicted to promote her growth in holiness, and her completeness in Christ. Not one suffering is lost (Ps 56:8). All her members have thus a mutual interest in one another's sufferings (1Co 12:26). But Rome's inference hence, is utterly false that the Church has a stock treasury of the merits and satisfactions of Christ and His apostles, out of which she may dispense indulgences; the context has no reference to sufferings in expiation of sin and productive of merit. Believers should regard their sufferings less in relation to themselves as individuals, and more as parts of a grand whole, carrying out God's perfect plan.
Matthew Henry's Concise Commentary
1:24-29 Both the sufferings of the Head and of the members are called the sufferings of Christ, and make up, as it were, one body of sufferings. But He suffered for the redemption of the church; we suffer on other accounts; for we do but slightly taste that cup of afflictions of which Christ first drank deeply. A Christian may be said to fill up that which remains of the sufferings of Christ, when he takes up his cross, and after the pattern of Christ, bears patiently the afflictions God allots to him. Let us be thankful that God has made known to us mysteries hidden from ages and generations, and has showed the riches of his glory among us. As Christ is preached among us, let us seriously inquire, whether he dwells and reigns in us; for this alone can warrant our assured hope of his glory. We must be faithful to death, through all trials, that we may receive the crown of life, and obtain the end of our faith, the salvation of our souls.

Seems like that "lacking" stuff may not be exclusively in the realm of Catholicism. Looks like a few folk on the other side of the fence just might agree on that one.

And then there is our FRiend...