I have an 88 2 door with a 5 speed and 3 inch lift. The rear is just blocks.

After driving for awhile there is a thumping metallic sound from under the Jeep. I can feel it in the floor. Usually happens during cruise. Doesn't seem to be anything other then random. But only above 35 miles per hour. Sounds kinda similar to binding in the 4wd. Transfer case has been spaced down for the lift.

The rear u joint has a fair angle to it. There doesn't seem to be shims to correct driveline angle. Plus the slip joint at the trans seems fairly loose, but not terrible. All u joints are tight, and show no rub marks. Jumping splines on the slip yoke?

How far should the forward u joint be from the end of the trans? Seems about 4 inches on mine. Also, not the factory driveshaft.

Or could it be the chain jumping in the transfer case? It frequently binds then jumps out of 4lo with a large bang.

Any other ideas?

Where would you start?

 

By checking to see if the exhaust is riding on the trans crossmember due to bad engine and/or trans mounts.

 

Mine was doing the same thing, at the same speed. Changed the tranny mount and problem solved! Try it!

 

Could be lots of issues.

Check your tcase linkage for the popping out of 4 low...

 

Randomly? Like clunk.......clunk clunk......clunk clunk clunk......clunk.....

No exhaust hitting. Not that i could tell.

Almost seems to jerk the rig to. Like momentary loss of power

 

Think I've narrowed it down.

Noticed a u joint chirp today the same time the sound happened. I'll toss some grease at them and see if it goes away.