Vibes @ 50 MPH

[jay79cj7]

I've posted this before, but never really figured it out and now its worse, so I'll try again...
I have a 00 XJ, 4.0L, auto, 95k miles, stock suspension and 20k mi old tires. (The CJ is for hacking up at this point in my life.)
When I hit about 50-54 MPH I have a vibration in the whole vehicle. It started in the steering wheel a few months back, but now I can feel it in my seat, and the back of the passenger seat really shakes around when nobody's in it. It is perfectly smooth up to that speed, and also smooth once I get over it. The vibes only occur at that narrow range of speed. Sometimes are worse than others. It seems to be worse when braking through that speed too.
It was just a nuisance that I didn't get around to with other projects and such, but now its getting pretty bad and I don't want it getting worse.

Since its coming through the steering column and the wheel is really bouncing around, I assume it has something to do with the front end and not the rear or drivetrain... I really despise the idea of taking it in, and want to figure it out myself, but its hard to crawl under when I am going 50!

Suggestions? Thanks in advance!

 

[indiemo]

I had this same exact problem and there are a couple things you can check. Check your trackbar at both ends and make sure it's tight. You can do this by watching it while a friend turns it all the way to the right and the left and watching the ends for play. Another thing to check are the control arms and make sure those aren't loose. The other, more pricey experiment is replacing the tires. That turned out to be the fix for my problem (after exhausting all my other options). The tires seemed fine but I must have had a broken belt or something in one or more of them. It could also be the ball joints so you might try jacking up the front end and see if there is any play in the front wheel if you put your hands at 12 and 6 and try to rock it. That's all I can really think of for now. Best of luck tracking it down!