Post-transmission mount blues- knocking when braking/decelerating
 

1993 Jeep XJ Sport 4.0 AW4 NP 231j with 4.5 rough country

Alright then, here's the skinny. I noticed a heavy chatter in the rear when i let off the gas at decent speed, so i submerged to find the offender. Found that the DS u joint at the pinion end was pretty lose and also found that my transmission mount was no bueno...So i replaced them. Easy fix, no bumps...happy day in the garage. I went to drive it around and could immediately feel the improvement and lack of the offending chatter (although the DS vibration did go up a tad, im guessing from the increased angle on the shaft after replacing the trans mount. I have no sye or drop kit on a 4.5 lift).

Then I went to braking and it happened...dear lord it was unnerving. There is a heavy, deep clicking in tune with the drive train rotation (in other words it slowed down as the jeep slowed down). I can feel it under my keester but I cant tell exactly where its coming from. The trans mount was bad enough that it really sagged the trans and tcase, enough the make a significant lift when i replaced it. Could that have revealed another issue? Or rather, it DID reveal another problem but what the hell is it?!? I double checked that my crossmember and mount were correctly installed and that nothing was bouncing off of it (exhaust) but the noise is not like that...its not a vibration or banging. Its something in time with the drive train, axle rotation. Sweet lord what is it????

When I first did the lift on my XJ I had the same exact issue. It is possible that the new mount just changed the angles a little bit and is causing the issues now. I would check your driveline angles and a sye/tcase drop/ shims should help you out (whatever your budget allows).

Here is a link with some info on measuring your angles too...

http://www.4xshaft.com/driveline101.html

Thanks for your input friend. Sounds spot on. I also think that not changing the motor mounts at the same time is further confounding the drive train angles...rookie mistake for sure. Gotta love when you fix one thing and two things f up...lol

are you sure the tcase isn't partially engaged or trying to engage?? it is under your keester. and would click if it is trying to engage as you slow down

The sound is a lot like the tire tread hitting the lip of the plastic fender. With the window down I can hear it right under me. I've done everything else imaginable to find it...then I remembered that I was f'ing around with the tcase shifter while it was parked and not running. I checked the shifting doodads and it's all very loose. Soooo I think you may be right, perhaps I'm stuck mid gear.

Do you have any advice on how to further check and or remedy? Thanks for your input!