So I've recently come across a strange vibration in my transfer case or at least I'm assuming it's in the transfer case as it seems to go away when I engage 4 wheel drive and accelerate. Symptoms include vibration at speeds over 45 miles per hour, vibration while decelerating, no vibration while in 4 x 4 at 45 or above while accelerating but still vibrates when decelerating. All motor mounts and transmission mounts have been replaced All U joints have been replaced and drive lines have been rebalanced so the question is what could be causing this vibration? Could a worn-out chain and or gear cause this vibration? The Jeep currently has 500,000 miles and to my knowledge the transfer case has never been opened it is also submitted to 4 wheel drive conditions daily as I live off road

 

I would check for play in the transfer case output shafts and the pinions on both axles.

Have you tried driving it without the front axle and rear axle (one at a time, obviously) to see if it's any different? Unless the older XJ's are different, it should be super easy to find a stretch of road that you can get up to speed on and take the axles out on the side of the road, test it and then put the axles back in.

It also wouldn't hurt to check your wheel hub bearings and ball joints. There could be looseness there that is being masked by the extra force when 4wd is engaged.

Edit - Also don't forget tires. I had a bad vibe at 55 and I chased it for weeks until I tried swapping out one of the tires. It was warped, I'm assuming since it was the original spare and was 12 years old at the time. It looked fine, I had it balanced and shop didn't notice anything wrong with it. The second I swapped it out though, the vibe was gone. I ended up having to put the tire on, get my inspection sticker and then take it off since the vibe free tire was bald and the warped one had a ton of tread on it.