Have you had your vehicle scanned for transmission codes. You may be looking up the wrong tree by looking at the ABS system? Have you looked at the transmission and transfer case and front and rear differentials? See if anything has been disconnected or not plugged in? You did have 3 rear diffs replaced, they might have forgot something?
Generally the ABS and the AWD system are two different systems. They may share some of the same sensors and may work in conjunction with each other (some vehicles more so than others) but like I said you may be barking up the wrong tree going after ABS probs.
I remember one other GM product that was having the same prob and I think the problem was an update for the tranfercase fluid and using a friction modifier?
Sorry I havent give you any conclusive ideas but maybe a fresh outlook may lead you to the answer?