Intermittent vibration in rear

2001 XJ, 4.0L auto, bone stock, 200K miles

Bought this XJ for my son to drive as his first vehicle. Really solid and drives great. It has one nagging issue I can't figure out - an intermittent vibration in the rear end.

It usually occurs around 45 - 50 mph. Its an audible vibration, like you have a huge sub-woofer in the back of the XJ. I mean everything rattles including the mirror. It usually happens when your accelerating at a moderate rate, but when it happens if I maintain speed it will keep doing it until I have to speed up or slow down. OR it can start and I keep accelerating and drive through it. It definitely acts like I'm hitting some kind of natural frequency of some part of the rear drive-line.

I've been under it looking for an issue and haven't found anything. It has new shocks, all the u-joints look good, and have no play. All the other bolts are tight. The only other thing I can think to check is tire balance.

I'm hoping someone may have had a similar issue and might give me some guidance.

thanks!
Clay

Two logical suspects are cheap tires and faulty u-joints.

Cheap tires can be out of round, or be internally out of balance. You should be using an independent tire shop with a Hunter Road Force Balance machine and not using a retail tire store with some kid fresh out of a tire balance training class. U joints can be going bad yet look normal. Typical faulty u-joints sounds are a vroooom, vroooom, vroooom kind of droning sound at highway speeds, or a sound that only occurs on acceleration, or only occurs on deceleration as the pinion angle changes under torque load.

Tim's advice is spot on.

I will add that to properly inspect the U-joints you really need to remove the driveshaft so that you can feel how they move freely, without any kind of tension or limitation. You need to be able to cycle them in your bare hands.

Having the balance checked on the tires is probably the easiest and most likely place where you should start.

Adding to the u-joints (you can't tell by looks) but grease them and also the splines on the slip joint.

Just to update this one - I've finally started looking into this one more closely. I pulled the drive shaft today hoping to find a smoking gun. So far the u-joints look find. Nice and tight and both freely moving. I will lube the splines for the slip joint and put it back together tomorrow.

Next move is to have all off the tires balanced. They're Bridgestone Duelers so not exactly junk tires, and they all have weights in place, but I'm hoping they find something.

thanks,
Clay

The most common issue like this I have found is the driveshaft, either out of balance or the centering bearing on the dual cardan joint is worn out and dry.

Thank Tom. This is a bone stock XJ so only single u-joints. That said while I have it out I can always have the balance checked.

Thanks!

Check the wheel bearings. Jack rear end up in the center, and try to shake both rear wheels.

I posted this in the other vibration thread, but figured I'd cross post here. I'll never go back to having my tires balanced on a machine.

"I also had a high mileage XJ (400K, got it when it had 30K) but it was a 2WD and I decided to upgrade last year to an 01 XJ 4WD. Anyways, I have always run Dynabeads and they are great. Going to get your tires balanced regularly is a headache where I live. Now I just rotate myself every time I change the oil. Huge time saver. The only downside is you need to keep some spare Dynabeads on hand in case you have to get a flat repaired. Dynamic balancing has been around a long time and there are other brands that are cheaper if you look around. The Dynabead site has some good info and also a chart to tell you how many ounces for a given tire size.
https://www.innovativebalancing.com/index.php "

well I took it Friday to get tires balanced. 3 of the 4 were out and I've had no more vibration issues since then. Fingers crossed that I'm done! thanks everyone!