Hi guys, I've got a rather long post ahead of me here. I've got a 1998 TJ that was purchased new by my brother in 98. It came with a 2.5, AX-5, and an NP231. The 2.5 threw a rod, so my brother swapped in a 4.0 and an NVG3550, retaining the factory NP231. The wife came, and the Jeep got old. The Jeep sat for approximately 2 1/2 years before I bought it from him. It was stock on 31's. I wheeled it, and occasionally heard a pop, it seemed to happen during articulation so I assumed it was a spring. This would ONLY happen OFF-ROAD in 2WD. I put 2.5" springs with .75" spacers on the Jeep, and also put on 1.25" Transfer case pucks to correct driveshaft angles. A few weeks later I heard the pop happen more frequently off road so I decided to replace the rear DS U-Joints. Ended up finding out my spider gears were about to grenade while checking the rear end, so I replaced it with a quality gear set. I backed the Jeep out of the shop, and the clunk happened again. So, I decided to replace the front DS U-Joints. I did drive it without the front drive shaft and the clunk remained. One thing I should note is that my linkage needs adjusted, because it really doesn't want to go into 4-HI but will function in 4-LO occasionally popping out. Fast forward, the clunk is happening every time I let out on the clutch, as soon as it hits the friction point allowing the front driveshaft to turn. (Still in 2WD) The front drive shaft appeared as if it were binding, so I chased it down the line and noticed that the needle bearings in the front axle U-Joints were non-existent. So, after having to red wrench the U-Joints out, I replaced them with Dana/Spicer. We are now at present day, after the U-Joints were installed it seemed to quiet the popping from driving forward, but sounds AWFUL when releasing the clutch to go in reverse. Under no circumstances will it pop in 4 Wheel Drive. I can drive it, and it will pop when starting and will sometimes continue to pop while going through gears up to 4th. I have checked the transmission mount, and it is not cracked.
Thanks in advance, sorry for the long post. Just trying to provide as much information as possible to help decipher. Before you ask, I have used the search function on numerous forums, and have been researching this constantly for a month with no success. I have found similar problems, where others had a loose chain causing 4WD popping. I have also considered that possibly the T-Case is in 4WD and the chain is loose, but the Jeep does fine as far as turning and on road handling. My brother says that he has never had any problems. *Note: All fluids were changed when I bought the Jeep from my brother in June.