You know I just thought of one other thing. Check your front axle (not drive shaft) u-joints. I had the u-joint on the drivers side front of my Ford wear out to the point where all of the needles were gone, and it would pop when I would make turns in 4wd.
If that were the case, it should pop all the time, even in 2WD, as long as the front hubs were not unlocked. If the front hubs are not unlocked, those axle shaft u-joints are always turning, even if the transfer case is not applying power to the front diff.
1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.