brian23456

It started a little over a year ago, vibrating/rumbling at about 70mph and above. I didn't look into it much since it rarely got up to that speed. It had gotten worse and started rumbling at around 55-60mph.
Since April, I have replaced the track bar and frame side bracket and drilled the other side out to 1/2". Front bearings, sway bar links, shocks, rotors, front driveshaft, steering stabilizer and a couple control arms (didn't do all of them, turns out they were fine). I did the axle ujoints and axle seals a couple years ago before it started getting crazy. Tie rods and ball joints look good, tried swapping wheels front to back.
So now I've just about ran out of ideas! I might go have the tires rebalanced, but I'm not very hopeful that will do it. The transmission mount is in rough shape but I'm not sold on that either.
It's a 2000 4x4 with stock suspension, the rear is all new stuff. Any opinions or advice would be very welcome!

jordan96xj

Rear drive shaft u-joints are certainly implicated. Have you checked/addressed it yet? A single dry/failing cap can rumble, and it will usually be in the 50-60mph range where it is most obvious.

I just trouble shot a new rumble that was developing in my 96 XJ and between 52 and 60 mph. I checked a lot of things. It came down to a single dry/failing cap on the rear most u-joint on the rear driveshaft.

Tires and alignment can certainly be implicated, but out of balance tires don't typically just start rumbling at 70 mph. Where as u-joints will typically rumble under heavy load, or at the point that the drive line is "gliding" (when you reach the speed you desire and then reduce power to maintain it). This is because u-joints are tight when under load, but once they are unloaded (and spinning fast) any play that they have will translate into rumble. This is why 55-60 is common for u-joint/drive-line problems, because that is were most people will be spinning fast, but reducing power to maintain constant speed.

Another source of rumble, and one that is often over looked is the sway bar to frame bushings. Especially the front sway bar. The road surface rumble will translate right into the frame and body, if these are not in good shape. They get hard and wallowed out over time, and any play in them will allow road surface rumble to resonate on the frame. However, this doesn't typically just come on over 70mph. But can, depending on the frequency of the road vibration.

Both are pretty cheap to address.

brian23456

I checked the rear driveshaft for play and there was none, but maybe it could still be the problem? I'll pull it and drive in front wheel drive tomorrow. If that doesn't do anything maybe I'll check the front joints, then break down and have the tires balanced.
The front sway bar bushings are new as well, forgot to add to the list!
Thanks for the reply!

jordan96xj

When I checked my rear drive shaft, there were no external signs of problems. No play was observed. But I had wanted to put a different type of u-joint in anyway, so I took the rear shaft out, and only when I started removing the existing u-joints did I find that one of the caps had suffered water intrusion and was devoid of lubrication. Given a few hundred more miles, the play probably would have become more obvious from the outside.

4WD4EVER

I would encourage you to not discount the effects of a worn-out trans mount. It may not be the entire cause, but, it sure doesn't help if your driveline is out-of-whack.

If you know its condition is questionable, replace it w/ a new Mopar mount. And check your motor mounts also.

Let us know what you discover.

brian23456

Pulled the rear driveshaft and it seemed fine, drove it up to 75mph and it was more or less the same. Tossed the driveshaft back in and had the tires balanced, it's a little smoother but it obviously didn't solve the problem. I took a closer look at the tranny mount and it looks like most of the play is coming from where the rubber would be glued to the metal base, it is separating. Could that thing really cause all that shaking I wonder? Motor mounts look excellent. The local stores want 100$ so I'll probably order one from rock auto and hope that helps. If I have time I might pull the front axles and check the u joints.

brian23456

Finally got a couple hours to work on it today! Used a ratchet strap to try and simulate a good trans mount and it was slightly better, might have been my imagination though. Pulled the front axles and the u joints on both sides were a little stiff in one direction, heated the yokes (?) and smacked them around until they were smooth. I did notice one thing with the axles and hubs on, one wheel one the ground, I could feel a very slight ticking. I couldn't feel it in the front driveshaft so I pulled it and could still feel it on the studs. Still not sure what it was. Normal front diff noises perhaps?
Put everything back together but left the front driveshaft and drove it down the highway. It was definitely better but there was still vibes, which seem to appear mostly under load or zero load at certain speeds, 35-40, 55-60 and 72-???mph. If I'm on cruise control at 73 (for example) the vibes almost disappear going down a slight hill, but are present on level ground or uphill, and worse if I totally let of the gas. This makes me think it's the rear driveshaft u joints, but they felt fine when I had them out. I might just have to replace them anyway, unless someone has any other things I should check out?