Hello fellow Jeep enthusiasts. I'm a new member and this is my first post. Glad to meet you.
My problem is that my 95 Jeep Wrangler has a vibration while underway starting at about 20 MPH. It's not felt in the steering wheel so I ruled my front end out. I had this problem before and the mechanic pulled the rear driveshaft and took it to the conveniently located driveshaft shop next door whereas it was deemed no problem. He reinstalled it and told me to go to 4WD specialist in town. On the way the there I found that the problem was gone. Lucky me. Well after about a year the same problem returned. I crawled under it yesterday and grabbed the same driveshaft and found that I can move it side to side about an inch or so (not in or out). I'm thinking this can't be right. Maybe there's some type of mount that I can't easily see that is loose or completely broken.
I had the same issue and immediately yanked the u-joint off. After buying the most expensive u-joint I could, I found the the problem remained. Turns out it was a $4 u-joint STRAP that had become worn down over the years and miles. I would suggest you go ahead and get both, just to save the frustration and the trips to the parts store.
I haven't yet replaced my U-Joints but yesterday I did pull the driveshaft out. The U-Joints seem fine to me. They're tight. No play at all. Damn. I'd hate to buy U-Joints only to find out it's something else.
I posed the same question on another Jeep forum and one reply suggested that I check my front driveshaft. I did more than that. I removed it completely. Unfortunately I still have the vibration. Another reply suggested I check the rear axles. They might not be straight. So That's the next step in this on going saga.