Romans715

I have vibrations at and above 45mph. When I hit the gas I can go up to however fast I want no vibrations. Let off the gas vibrations occur... If at or above 45mph. It's 4.0 i6, aw4 tranny, 242 transfer, d30 and d35. 1994 Jeep XJ country with 3 inch lift and 31x10.50 coursers. Happened before the larger tires. Front driveshaft u joints feel good and everything is greased. I have new gear oil in front axle. I have gear oil and rtv to do the rear. I'll check rear u joints while I am there. Any ideas. Thoughts. Or solutions? Thank you all.

kbeam418

Check the motor mounts and transmission mounts, I owned a car that had the same exact symptoms and that's what it was. One thing to keep in mind, imo, is we're driving vehicles that were designed more for utility than comfort so some vibes is to be expected. It's a solid axle, high center of gravity vehicle with a stiff suspension so you'll never get a smooth as glass ride. Reason why I'm saying this is I was obsessed with trying to get mine perfect until a mechanic finally told me there's nothing wrong and to just enjoy my xj.

gat

My 1992 had a compressed mount for the transmission that I replaced. Some time after that the threaded bosses in the block of the engine broke where the three bolts for the transmission mount attached. I replaced both transmission mounts with some custom solutions that use all the threaded bosses from the factory instead of just three. When you look at your motor mounts ensure that your brackets are not cracking the bosses of the block or missing bolts. There are three manufacturers of these custom brackets. I think that they all come with custom mounts, too.

PatHenry

Definitely some good advice by the previous posters.

Only thing I'll add is you might consider doing a transfer case drop. At 3" of lift the driveline geometry will be slightly out of whack and lowering the t-case can bring it back into a more smoothly operating position.
I would only be concerned if you are having vibes only in a specific gear - but on an auto I'm not sure if even that is really a factor.

Romans715

Ok thank all of you. I did replace my engine motor mounts over the summer. I will check out my tranny mount see if it needs replaced and I will definitely buy the drop kit, I've read it sometimes is needed. Is there a specific brand or part # you would recommend?

PatHenry

If you really want to spend money, you can get a SYE and put a CV rear shaft on it. However, the easiest way is to grab a few thick grade 8 washers and put them between the crossmember and the frame.

Beyond that, I have no idea what kits might get you. There may be ones to deal with the fact there are studs and bolts for the crossmember.

I'd start with washers, especially if you are taking the crossmember off to replace the Trans mount. That way, you don't spend a ton of dough and you can see how it affects the vibes.

Romans715

Thank you Pat! You have helped answer many of my questions!

PatHenry

Anytime man! Keep us posted on how it goes!