If its the xfer case and not the front some idiot tried to drain the case fluid from the detent ball plug. Just take out plug and see if spring and plunger are in it still. This was the problem on the jeep i bought. It would pop out of 4wd under a load cause the detent ball was gone. If its gone u can get a new one for under 20 bucks.
89 YJ Islander, AC,PS, 10 inch lift, 32 BFG MT's, sway quick disconnect, rear track bar quick disconnect, 400 watt 12 speaker cd player for tunes,8000lb warn 8274 with 150 ft cable, and a high lift jack.. Lockers, 35's, webber carb and jacobs ignition comming soon.