A vibration I just cannot locate...

I bought my XJ about 2 months ago. its a 97 Base(ex hwy dept). 4.0L Automatic. NP231, Chrysler 8 1/4. 200k miles. 3" rough country lift with extra 1" blocks in the back to keep the door stays working. I have this Vibration that I just cannot figure out its origin. Before I describe the vibration, I just replaced the entire rear end due to a bent axle and some really nasty differential fluid. I also installed 2 new U-joints in the rear shaft. This is when I added the blocks(I know, blocks suck BUT i was sick of the doors hitting me before i could get out). None of this work had ANY bearing on the noise, with the exception of the Ujoints, I had one that was borderline and the new joints defined the noise a lot better.

I have also tried 3 different sets of balanced wheels and tires of various sizes, none of which change the characteristics of this noise.

The Vibration occurs around 30mph and then again from 55mph to oblivion. It isn't necessarily a rotational vibration, it is more of a harmonic overtone that you can hear a lot stronger in the front part of the cab. Its a rumble you can hear and feel in the floorboards. You can't feel it in the steering wheel or shifter/T-case selector
The noise at 60mph is about twice the pitch at 30 and half the severity.
The noise IS load related. the Noise during float is much less worse and definitely at its worst at WOT and Decel.
Turning side to side during vibration does not change the way it sounds
When you hit bumps, it kind of creates a similar noise and "interupts" the vibration.
When riding in the back, the noise is much less worse.
When vibrating, if i pop the transmission into neutral the noise is the same as if i let off of the gas. If i rev in neutral while moving the noise remains unchanged.
The noise is identical, i mean IDENTICAL in 2wd and 4wd.
When doing a 1-tire fire in the wet(yeah, i know, bad idea) the noise occurs with the same characteristics as the spedometer indicates.
The noise gets worse the longer you drive. When you first start it and when the car is relatively cold, the noise is about half as worse as when the its been on the road for 30min.

Also:

When at a stop and taking your foot off of the brake, before you hit the gas, the vehicle chugs as if it misfires. It does not misfire. the noise is gone by 2-3mph but also acts the same in reverse. It will also do it in Drive when rolling backwards on a steep hill. It does not chug at a dead stop. This might be a completely unrelated issue.

I have also swapped rear driveshafts thinking mine might be bent, however this rendered NO difference.

The noise sort of feels like a major drivetrain component is against the body, but it is not. I have checked all the engine and transmission mounts and all are in acceptable condition. The Exhaust while having one broken clamp does not contact any part of the chassis.

The front differential leaks from the pinion seal, I keep fluid in it but it was rather low when I bought the jeep. Haven't had time to fix it but since the noise is the same when the front diff is engaged and when its not then i don't think this has anything to do with the vibration.

I was leaning towards a possible transfercase issue but i would think the noise would be different in some way when in 4wd compared to 2wd. its the exact same, which also makes me want to rule out a front Ujoint or differential.

I thought, perhaps i have a torque converter lockup issue. But this would not explain why when i put it in neutral the noise does not change.

I'm not sure if the lift has anything to do with this noise. installing the blocks did not change the noise at all, which makes me think it does not have anything to do with the lift.

I have a 79k mile Transmission and Transfercase but i REALLY don't want to throw parts at it trying to fix it. Thats something dobbs does, and I'm not dobbs.

Any help would be GREATLY appreciated!

-Eric

Search for slip yoke eliminator.

I had varying harmonic vibrations upon acceleration and braking and all the time between 25-35mph after my lift, so I did a SYE... use the yoke off the stock driveshaft to help you center the pilot hole. I was super careful and did it without vibes. Also while cutting the output shaft use a grinder and cutoff wheel, run XJ in 2hi reverse *ebrake, chock wheels, everything!* and it takes a few minutes and produces a nice, clean cut with a dimple in the middle.

alright
so what are your rear pinion and tcase angles?
with a stock setup(no sye) your pinion and tcase output angle should be equal, maybe have the pinion 1-2* below the t case output.
do you know if there is reason to suspect your rear output shaft is bent???
what are youre front pinion and t case angles
have you opened up the front diff to check it out?
have you measured and installed shims???
have you replaced or at least checked all control arm bushings?
checked all tre's?
done a good alignment yourself?
checked trackbar for play?
pulling each shaft one at a time will tell alot.
so you have 4" rear , 3" front with a 97 tcase?
consider the exhaust as well.
i had two of three broken exhaust clamps, that made the jeep sound terrible.
weld up that exhuast mount and see if that helps.
and youre sure your current tires are freshly balanced?

JJacobs said:

Search for slip yoke eliminator.

Click to expand...

X2. The newer XJ's do not like much over 3" of lift, some do not like lifts at all with no SYE(slip yoke eliminator). The way the transfer case is, at that height puts to much stress on the u-joints and the transfer case bearing and housing and can create bad vibs.