I have had this xj for 6 months and I have had this sound for a bit but I really want to figure out the cause since I have more money... So I think it started when I lifted the jeep. Whenever I accelerate I hear a slight kick and grinding sound and the same when I let go of the gas. This only occurs at speeds higher than 30... When I am between 30-40 the Jeep makes a low droning sound kind of what you'd assume aggressive tires to sound like. I have two things in mind (u-joints or transfer case drop) but i would like ya'lls opinion.

 

It should be easy emough to check your u-joints, so that's a good idea. I'd double check your transfer case fluid level (actually, I'd just change it to be sure) and the check the diffs and tranny while you're under there to start.