Won't move when the transfer case is in 4WD

Like @BLACKJKU said, transfer case. If it rolls in 2wd you can rule out binding in the ring and pinions. Also sounds like you can shift fine at speed? So the transmission is okay. Only thing left is the transfer case.

Next step would be to pull the front drive shaft and put it in 4WD. If it still binds up, you'll know with certainty its the transfer case.
You wont know whats wrong with it until you break it open unfortunately. Likely the chain but wont know until its open.

Scott