Weird vibration

The jeep has picked up a really weird vibration when letting off the gas slowing down in the drivetrain. At first I thought it was acoustics from aging tires but I rotated them and hung my head out and it seems to be coming from the rear portion of the jeep. The fluid in the TC is full. My next step is to put more grease in the wheel bearings and take the cover off the tranny. Has anyone ever had that rear transmission nut get loose and cause slack in the gears? Reason I ask is the synchronizer between 1st and 2nd doesn't seem to be working anymore as it grinds and clunks into gear now.

Just as a side note, everything on the jeep that moves inside and out is new from 12k miles ago when I did the rebuild and has never been deep enough in mud to get into anything

 

Rear universal joint going out?

 

Yes, I've seen that nut come loose in the 69 Commando I scrapped out, seemed to cause a bit of grief inside the t-case to the input gear.

 

Yep, vibration on decelleration has been a u-joint for me.

 

Definitely rear driveshaft related. Either sloppy t-case output bearing, rear pinion bearing, worn u-joints, or - this may be an odd question, but have you had the telescoping section of the driveshaft apart? If the splines are put back together in the wrong place, it creates a disturbing vibration, especially on deceleration.

 

If you have a drum type parking brake, the linkage can cause a weird vibration sound on de-celeration.
Happened to me. Yanked the pull handle hard a couple of times and fixed it..

 

And the winner is............................rear U joint. One of the 4 joints had probably 6 or 8 pulverized needle bearing and the cap was full of what looked like black ash and no wet grease. It was the one at the rear of the TC. That joint was stuck in one position.

Thanks guys....didn't even occur to me since everything was new. Let this be a warning; grease those U Joints on a regular basis. I only did it maybe once since original install.

 

Glad it was simple and cheap. Thanks for the update.