Mystery Vibe...

OK...gonna be a long explanation, but I feel necessary for clarity.

First vehcle info...'89 Wagoneer LTD, 4.0L, NP242, AW4, 3" Skyjacker Lift, AA SYE, LP D30 non-disco front with 4.56's (Yukon Gear) and Eaton E-Locker, Ford 8.8 rear with Ected LS/Locker w/4.56 gears (Yukon Gear), 33x10.5 BFG AT's.

I've got vibes that are the right frequency for driveline. Start about 40 mph...get worse the faster you go. Before I did the gears/lockers, had a transfer case lowering kit...no vibes. Busted a rear DS u-joint at the rear axle yoke and was towed the 6 miles to home. Put in used rear driveshaft and used jeep (with vibes) for about 2 months, then began the rebuild to install gears/lockers/new rear axle, figuring vibes would disappear with so many parts being replaced.

After installing new axle in rear (with all new internals except axle shafts), and installing new differential carrier and ring/pinion in front, and installing new SYE and new rear DS, had vibes that would put your right foot to sleep at speeds over 50 mph. Started to try to find vibes.

Local shop rebuild and balanced front DS...no cure. They re-balanced it (was not done right the first time)...still no cure. They checked and found my new Tom Woods rear CV driveshaft "a bit" out of balance...still no cure. They found "frozen" front axle u-joint, rebuilt both...still no cure. After spending $1,000 on their attempts at a fix, I decided to diagnose on my own...I'm no ASE mechanic, but know my way around.

First, I put rear axle on jack stands and ran in 2WD to about 60 mph...minor vibes. Got under it while in drive at idle (about 5 mph)...found that the rear driveshaft "wobbled" in the middle. Found this by resting a long screwdirver on shaft...was not smooth, showed shaft was "out of round" by about 1/16". Axle flange was true...transfer case output flange was also true. Figured rear DS was either bent or hub-centric flanges weren't perfectly centered at mating surfaces.

Then, removed rear DS, jacked up front and ran front only up to about 60 mph. Minor vibes again. Best as I can tell, front DS is true...no wobble. Test drove with just front DS...vibes there under power and coast, but less inbetween.

Then removed front DS too. Ran in garage with no DS's connected...up to 60 mph...no vibes. Re-installed rear DS and went for test drive (no front DS). Vibes noticeably reduced...just minor high speed vibes.

Back in garage...looking underneath to find culprit. With front wheels on the ground and turning the front diff yoke pinion by hand, can see inner axle ears move without any movement of outer ears. Bad axle u-joints? I think so, but shop just rebuild them in their search to find vibes!!!! Also felt a "slip-grab" when turning front pinoin...maybe bad ring/pinion contact?

Here the dilemma...I know (think) that the rear DS is not true. It's causing minor vibes due to "out of true" condition. But about the major vibes...obviously from the front. Can bad axle u-joints cause this? Perhaps bad ring/pinion contact? Should I send the Tom Woods DS back for "re-trueing"?

If I have to rebuild the front axle u-joints anyway...I'll think I'll get JY later model shafts with the stronger (290x) u-joints, install new joints, and make my current axles my spares. But...will this cure my highway vibes.

Expert advice is desperately needed...don't want to replace parts needlessly. Thanks in advance for putting up with the long post...

Flyfisher:wave:

OK...bumping this with update.

Went to my favorite (maybe not for long) shop. Guy said the slight movement in the axle u-joints was normal. Mmmm. They are sending the front DS out for a re-re-rebalancing to check it. Otherwise, they're thinking front wheel bearings, or even differential carrier bearings.

Any of this make any sense?

Well, you lifted it - the front driveshaft, since it has a double cardan at the TC end, should have nearly zero U-joint angle at the differential end. Anything else will cause slight vibrations from the spinning wheels turning the driveshaft, I would think. Unfortunately the only ways I can think of to correct this without cutting the welds holding the axle tubes to the pumpkin will throw your caster all out of wack, as well as possibly requiring remounting your spring perches and track bar mount...

Another update. Tired of waiting on the results of the third front DS rebalance...I installed a 20-year old front DS from a parts Jeep. Amazingly...seems to work pretty good! Seems to confirm that the front DS is the culprit! Can't believe it...after a rebuild..and two balancings, but the "seat of the pants" test proves it.

Will chat with the shop tomorrow..will let you know.

Also,you didnt mention any driveline angles(front or rear)!

Your vibe from the front is most likely made worse by bad pinion angle I am in the process of rotating the inner "c"s on my front axle due to bad pinion angle causing vibrations at high speed.

Haven't measured the front driveline angles, but rear shows a 2.5* difference from pinion to driveshaft...good.

Lift (3" Skyjacker with new LCA's) has been on the XJ for over 5 years (with a TC lowering kit). Most recent upgrades (see OP) included removing the TC lowering kit since SYE now installed.

Haven't heard what mechanic has to say about other front DS yet...should be able to talk to him yet today. Will keep all posted!

Flyfisher said:

Haven't measured the front driveline angles, but rear shows a 2.5* difference from pinion to driveshaft...good.

Click to expand...

Not good. You want the pinion to be 1* lower than the driveshaft.

Also, axleshaft u-joints should have 0 play.