Join Date: Dec 2001
Location: Belleville, Illinois
Thanked 18 Times in 13 Posts
A very clear description of the problem. Good job!
Play in the U-joint tabs can certainly cause vibration, and hitting bumps could make it come and go. The cross can slide freely in the caps, so a bump while it's turned a certain way could slide it off center causing vibration, and a bump when it's turned the other way could slide it back into its proper position. In any case, you should fix the problem.
A dial indicator riding on the driveshaft tube will show if the shaft is running true. Thump the shaft with a dead blow hammer to move it to its extremes in the yoke. Any run out will cause vibration. If you find runout you may be able to shim it out, but eventually you should replace the yoke.
If it comes and goes in a steady cycle it suggests that there are two components vibrating. When they are in sync you feel it, when they drift out of sync, because they're spinning at slightly different speeds they cancel each other and you no longer feel it. For example, if the front driveshaft were spinning with the tires, it and the back shaft could turn at slightly different speeds due to different tire sizes or different gearsets in the differentials. Since you've stopped the front shaft with ties it can't be that, but bent axle shafts could cause the problem.
You don't mention if your CJ has stock suspension, but a lift can change the driveshaft angles in a way that causes vibration. Normally one would expect it to be constant at constant speed, but old Jeeps have a way of doing things not logically possible. And the same thing is possible if the power plant is sitting slightly sideways in the frame, although I would think the angle would have to be extreme for it to show up.
Last edited by Jim_Lou; 08-11-2015 at 04:12 PM.