Vibes at 45 MPH

2001 XJ Sport, 164k mi, 4.0, AW-4, SYE, D44, 8.8, 37" MTRs

So driving home from the meet and greet tonight I was reminded of a vibe I'm trying to chase down. Seems worst around 45 MPH, smooths out some after that, starts to get a little noisy again above 70. Also, the second I take my foot off the gas it stops.

Front hubs are manual and in the open position, so no front driveshaft movement. If it was the tires I wouldn't think it would stop when I let off the gas. I've had an alignment recently as well.

The only thing I can think of is the rear driveshaft. I had the splines and yoke/sleve replaced when it was lengthened, and it was reballanced. Then on a road trip the socket/ball piece in the double cardan joint went bad so I had that replaced and the U-joints. All of this in the last 600 miles or so. (Note to self, don't lengthen a stock JY shaft, just buy a custom built one!)

Anyway, if the rear driveshaft just needs to be reballanced for some reason then cool, but is there something else I could be overlooking? Also, I've had 2 of those ball and socket joints go out on me, both were JY shafts so maybe that's the problem, or could something upline be causing the vibes and therefore the failures? (The previous owner did the SYE, but looks good and fluid in the transfer case is where it should be.)

Any insight appreciated!

Could be the rear axle pinion to driveshaft angle needs adjusting.
Is the rear driveshaft pointed straight at the pinion or down a degree or two?
The vib stopping when you let off the gas sounds like the pinion is too high.
It's where I would start looking

Ya don't say... Well, when I was setting up the brackets on the rear axle I put the jeep on the springs and jacked up the pinion to point straight at the transfer case. Of course I'm a genius and didn't have the tire in the trunk or any tools, gear, etc. So my pinion actually points a few degrees HIGHER than my driveshaft. :sad1:

Here's a pic:



So you're saying that could cause it ... Well, I have some leafs lying around I've been meaning to put under it to hopefully give me another inch in the rear, that should help some, but what is ideal ~ 1-2° lower than the driveshaft angle?? I may need to get some shims...

check the u joints, and you will have to reassess your angles if you lift again. Ideal angle is five degrees lower to compensate for axle twist.

should be within a degree or two of each other between, axle yoke drive shaft then t-casse yoke to double cardan to shaft....a socket that fits level on the u-joint and an angle finder

ProjectGuy said:

check the u joints, and you will have to reassess your angles if you lift again. Ideal angle is five degrees lower to compensate for axle twist.

Click to expand...

:doh:



No.

G2WANIT said:

So you're saying that could cause it ... but what is ideal ~ 1-2° lower than the driveshaft angle?? I may need to get some shims...

Click to expand...

No wonder you have vibs on acceleration.
Your pinion angle is pointed above the driveshaft when it should be below 1-2 degrees. Probably should start by lowing it 4 degrees or so. Shims are the easy way to do it.
http://www.4wheelparts.com/Lift-Kit...Spring-Axle-Shim.aspx?t_c=1&t_s=101&t_pt=5502.