I have a vib when I get above 55 normally and whenever I let off the gas and hit the gas again I can feel the rear jolt and make a clunking noise. I have read somewhere that the u joints can cause these problems so I checked them out and they look ok to me but im not 100 percent sure I checked them correctly. They look almost original and the xj has 220k on it so could this be the problem?

 

Unless they are really bad where you can feel or see the slop, pulling the driveshaft and moving the joints by hand is the best test. Should operate smoothly, no stiff spots or looseness. It isn't uncommon to have them appear just fine until you remove them to check.

 

I cant feel anything abnormal when moving the joints and driveshaft. I guess it wouldnt hurt to change them anyways considering the mileage of the jeep.

 

That's the thing, lol. I just changed halfshaft joints on a different car of mine. Vibration, a little clunk here and a little squeak there, but when I grabbed them and twisted they felt fine. Nice and tight.
When I pulled them out and moved them by hand I found they were rusted nearly solid!