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Pull the fill plug out of the rear end, and the transfer case. Stick your finger in, and pull some oil out. Wich ever one has metalic in the oil is where your problem is!
Does the truck have auto locking hubs ?Sometimes they don't disengage properly .
I really suspect the auto locking hubs . I have the same truck and after I shift from 4 to 2wd I get a lot of noise from the hubs (grinding popping ). I usually have to back up for quite a ways to get them to release ,and it usually takes several times . You and I both would probably benefit from a manual hub swap and a rear end rebuild .