Job 38:1-7
My conjecture is that in the Old Testament, angels are sometimes the messengers of God, but sometimes also they are God Himself speaking.
We are told in Exodus that no one can look upon God and live. So the Jews speak about the Shekinah, the manifestation of the Glory of God, but not the actual appearance of God himself. Such are the pillars of cloud and fire in Exodus, similar to the ones that appear between the divided sacrifice of Abraham in Genesis.
When the Three Young Men appear to Abraham in Genesis and tell him that Sarah will have a son before a year has passed, before they go down to destroy Sodom and Gomorrah, the text sometimes seems to speak of them as angels, and sometimes as God himself. Take a look at the text and you will notice this. The later Christian interpretation is that the three young men represent the Trinity. The Jews believed in the existence of angels, but sometimes the Bible also uses angels when speaking about God himself as He appears to the prophets.
Yes, I would equate the Shekinah, or presence of God, with a presence in the pillars.
And yes, the three who appear to Abraham and Sarah is a difficult passage.