Be honest. Post the Melania ad from the day before right above that Trump retweet. Then it makes perfect sense.
The Melania ad was a cheap shot gutter-politics tactic. But Cruz didn't run it. If I understand it correctly, Trump really did tweet that comparison between his wife and Cruz's wife.
Now you may argue that Cruz was behind that PAC ad, but he says he wasn't.
I can personally attest to the fact that when I was politically active, My group would run ads directed at the opposition which the Republican candidate had neither input or even knowledge of. I and my group were third party participants, and we hammered the Democrat candidate into Dog-Sh*t.
We even received a notice from the Federal Election commission informing us that the losing Democrat Candidate had filed charges against us, and that we were to appear at a hearing to answer them.
My point is, third party attack ads do exist. In Mormon Utah, I'm not surprised that they pushed a specifically Mormon perspective on attack ad against Trump, who let's face it, doesn't appeal to the religious as much as does Ted Cruz.
I find the possibility that a third party, religiously oriented advocacy group, would run such an ad to damage Trump, to be very plausible.