i don't think its anything bad.
the 4x4's i have messed with all made a slight clunking sound with it went from 2hi to 4hi.
i don't think that its the motor, from what i could tell the clunk was just the transfer case switching over.
but if its really loud then it could be the motor, or something worse.
I could see how a high mileage highly used 4x4 Tcase could clunk. I mean alot of trannys clunk as they get old. Personally I would see if you could get some SeaFoam in there and run it out. Im fairly sure they make tranny and tcase seafoam. It works great on noisy valves and rods and everything else.