Popping in 4 Wheel drive

Were you testing the 4 WD on dry tarmacadam or concrete and operating the truck in a relatively tight turn? It could be natural for the 4 WD to chatter, that is why you must not lock the 4WD unless the road surface is slippery. Locking the 4 WD and using it in this manner puts stress on the 4 WD components and can cause damage.

Check the u-joints coming out of the transfer case. While under the vehicle, grab the drive shaft that connects to the front axle and move it back and forth (with e-brake on or in park) and check for any play or movement in the joint.

The same thing happened to me. I bought a 93 yj. when i was in 4wd, it would have a steady pop when i turned. still never figured out what the problem was, but i put a new transfer in that had the slip yoke eleminator and it never did it again. Im guessing it was my transfer. i still havnt pulled the old transfer apart yet to see why exactly. Before i put the new transfer in, i also changed my u-joints, tie rod ends, pitman and idler arm, pulled my axels and replaced axel seals and it still popped when i turned. when i pull the old transfer apart ill know the real answer why, but i think the chain was stretched and all the stress from the tight turn made it slip. Thats my guess