I posted this yesterday in a different spot but didn't get much input so hoping someone will chime in. Here's the deal, saturday I replaced my broken wedge. I did some math prior to ordering and decided to go with 8* rather then the 6* that was on it( I have 8+ inchs lift). Immediately after doing so I went for a short drive and all was good, hit the highway and still no issues so I came home. Yesterday I go out to work on a friends house and about 20 min into the drive, going around 40, the jeep starts shaking all over the place. Never did that before. Could the difference in angle of the shims be causing this? Could the rear driveline be to straight? It has been mentioned that this could be bumpsteer so here are some pictures of my front end, not sure if something is out of wack or not so let me know if you see something.

 

Im no mechanic but i believe the u joint angles on either side of the drive shaft are supposed to match or it can result in the vibration you describe. The 2* change would alter the pinion angle while the other joint remained the same because the ride height didnt change. There are a lot of people out there smarter than me on this so hopefully someone can help if i am way off.

 

You have described the DWobble. Drag Link and Trac Bar, draw a line eye to eye, ideal is parrallel. Your upper end drop track mount helps and yours do appear very close. Can have an influence. Check all ends and joints, need to be in good condition.
Often in lifts there is not enough caster dialed back in after the lift. Need the lower control arms forward to rotate more caster back into the axle assembly. Are your lower arms adjustable or can you shim at the back. There is an excellent reference table on a site Gojeep.xj.info, at sometimes the site is not accessable, keep trying, it has great XJ build info. If you have an angle finder check on level ground using the heads of the bolts on your diff cover, diff should tilt back 2.5 - 4 deg from vertical as a reference. Measure your tire to frame clearance on both sides, if passenger is closer the track bar needs more length to allow the axle to center to the frame. Good luck, lets us know what you find out.

 

Good info, it should be noted that I have driven this rig for almost 8 months on road and off as well as highway driving and never had an issue until now. The only thing that I have changed is the rear axle shims so that is why I am leaning toward them being the problem.

 

Unless your bushings just **** the bed...

 

I see what you mean. I should have read twice. You probably checked to see the universals are not pooching out, wheel nuts all tight. Are the wheel bearings in good shape. I see a lot of the bearing hubs in front ends failing in many vehicles thru our shop. I have had a rear bearing failure develop on my XJ leading to a vibration although it develoepd gradually. Perhaps also check the leafs for break. I had an old camaro onece that instantly developed a vibe, searched for couple weeks then found a failed leaf cracked and broke but still held in the pack.

 

PS, the 2 degree change may have had an effect, I have less lift than yours but a 3 degree change did a fix to my rear vibe on this XJ.

 

Anyone know if this is true. Has changing the angle of the rear 2* and not changing the front caused some sort of issue?

 

http://www.4xshaft.com/driveline101.html

Heres an example. Again, ive never dealt with it myself but i remember reading about it a while back. I also would like to read some words of experience on this...

 

your front a rear drive shafts don't have to operate at the same angle to avoid vibrations... but they angle does have been pretty low.....

let me ask you this... Are you running and SYE kit on your t-case... and also, you may want to check into upgrading your steering setup to something aftermarket... Something that has a 1 piece steering link from knuckle to knuckle and then a separate link that connects from the Drop pitman arm down to the steering link.

 

I am running an sye, and i have thought about upgraded steering even prior to this issue. I am not really sure what direction to go with that. I saw a member on here was doing the serious 1-ton steering so I am interested to see how he feels about it. I haven't had a chance to do much with this issue, but I driven this rig every day and haven't had it happen again, of course I am not getting over 35 either.