To start off here are the specs on my jeep.. 4in suspension lift, sye, cv driveshaft, adjustable upper and lower control arms in the rear. I have the pinion angle set to about 1 or 2 degrees lower than the driveshaft angle (as is recommended). The vibrations occur from 30-50 mph and cause the whole jeep to shake. Im pretty much at "wits end" with this thing and need some suggestions on what to try next. 
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Keep it under 30 MPH ;D
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doublecheck the front stuff... tighten the control arms, trackbar, etc. if it happened shortly after the lift install, that's where I'd start. a beefy steering stabilizer will help mask it, but the problems will still be there.
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Check tire balance and alignment.
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lately his jeep has been throwing u joints, usually about once every three months, so we are pretty sure its from the rear.
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First try here...
http://www.rubiconownersforum.…iewtopic.php?f=38&t=29388
Trackbar and lower ball joints are good places to look. And don't forget about your tires. Try another set of similar size just to see if you still have the issue.
EDIT: Control arm bushings maybe? And not to be ignorant but is the rear ds the correct length? And regarding tires, out of round tires will balance perfectly. Just keep that in mind...
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i just picked up a set of lowers because my old control arms had bad bushings. didnt fix it
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Check the pinion yoke on the rear, make sure it is not egged out where the ujoint is clamped in, I have seen these egged out causing vibrations even in the steering wheel. This could also cause premature u joint wear. Hope this helps.
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The vibrations occur from 30-50 mph and cause the whole jeep to shake.Is it to the point where you have to slam on the brakes to get it to stop shaking ?
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is the vibration in the steering wheel or driveline do you feel it in your hands or the seat of your pants. if its in the seat you can remove the rear shaft and run it to see if vib.is gone if not pull the front shaft and try it.notice the change and go from there
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i just picked up a set of lowers because my old control arms had bad bushings. didnt fix it
Damn
You said something about your rear drive shaft. Why not take it off and see if you still get vibration? If you are blowing rear drive shaft u-joints as Chris said, I'd bet there is some binding there somewhere. Good luck man. -
1 or 2 degrees lower than the driveshaft angle (as is recommended). The vibrations occur from 30-50 mph and cause the whole jeep to shake.i am pretty sure that is with leafs in the back only because the pinion will rotate under load unlike your TJ unless you have bad arm bushings, every TJ i ever did the pinion was right in line with the driveshaft and i have never had any issues. i never use a degree finder, i just eye it up!
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i thought you did that to keep the u-joint lubed up. if its perfectly straight the caps would never spin.
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i am pretty sure that is with leafs in the back only because the pinion will rotate under load unlike your TJ unless you have bad arm bushings...you're kidding, right?
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check the output on your transfer case Ive known several people to bend them and have the same problem.
pinion angle all depends on the style drive shaft you are running a cv style shaft should be run at 1 to2 degrees below on a tj and a yj should be 3 to 4 degress below
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pinion angle all depends on the style drive shaft you are running a cv style shaft should be run at 1 to2 degrees below on a tj and a yj should be 3 to 4 degress below
Is that because of the suspension differences, e.i.: coil vs leaf?
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leaf springs hold the axle in position...and also provide the articulation of the axle. so when you accelerate they (leaf springs) will flex somewhat since that is their inherit design.
coil springs don't hold the axle in position - the control arms do. but there is still some flex given by the control arms upon acceleration due to the ends of the control arms and by the squatting of the tub on acceleration since repositioning the tub will also affect pinion angle.
in the end, you need to allow for more flexing with leaf springs than coil springs...but both require a downward position of the pinion in relation to the transfer cases output shaft. try aiming your pinion shaft somewhat above your transfer case output shaft and you'll quickly prove the point....unless you drive everywhere in reverse!
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i thought you did that to keep the u-joint lubed up. if its perfectly straight the caps would never spin.no, the offset of the pinion keeps the bearing spinning.
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you're kidding, right?
no i'm not, i have run any and all jeeps i built that way and never had a failure of a u joint. Like i said the offset of the pinion on the housing keeps the bearings spinning, perfect example which is easy to see is the 8.8, the pinion is very noticably to the one side.
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i think we are talking about 2 differant things. i am talking about rotating the axle 2 degrees lower than directly at the t/c to keep the needle bearings spinning i.e caps spinning
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