mysterious vibration or harmonic
I have a 1992 4.0 4x4 XJ sport completely stock suspension and drive train except for Goodyear Wrangler 235-75-15's and 2" coil spacers and rough country AAl in back.
The jeep had been operating fine for 6 months on these modifications steered straight, rolled smooth and quiet just like a new stock jeep would.
Then 4 days ago about a mile into my 18 mile commute as i took off from a stop sigh it shuddered like the emergency brake was on through first and second gears. It smoothed out at between 20 and 45 mph and then at between 55-60 i feel a vibration that cycles like a harmonic. I have checked tire pressures shaken all four wheels and there appears to be no bearing play. tierods feel tight as does the U joints at the front wheels and in the driveline. The condition seems to be getting worse and the vibe mostly goes away when I disengage the clutch and coast though not totally. It also does not appear to to be in the brakes either after my commute home tonight i felt of the rotors and drums and they were cool to the touch. Suggestions, Thoughts, Any Ideas at all will be appreciated.

 

Well a bit of news I engaged my part time 4high on the way home today just to see if it impacted the noise or vibration. 2 hi I get the vibe or harmonic under power but it goes away by letting off the gas or depressing the clutch. In 4 high on smooth pavement it sounded like I was driving over the wake up bars on the inside of the shoulder of a highway. Again this occurred under power and went away under coasting conditions. I am going to check all the mounts this weekend but I really suspect from the vibe I feel, that it is rotational in nature. It really feels like a wheel bearing but ours is no regular inexpensive bearing but a hub bearing assembly that is a bit too pricey to replace on just a guess at this time.

 

having the same problem in my 94 5speed 4.0
from about 20mph to 45/50 i have an extreme vibration. not death wobble just a vibe. no idea what it is. do u have the alloy wheels or steel?
i have the alloy and they tend to caroad behind the wheel and on the mounting surface of the disc and drum. did that and stopped my death wobble. havent tried it for this vibe yet.

 

my 99 gc was doing something like that. After checking everything with no avail,
I dropped the driveshaft and found that the bearings in the u-joint were dust.
Couldn't tell by just prying on it, trying to make it flex.

 

I'd be checking the u-joints in the rear driveshaft.

 

I have similar issues. Lots of noise & vibes, especially a thumping that couldn't be pinpointed.
My thread got no replies. I'm glad you're getting some.
I've rebuilt my rear driveshaft, replaced the loose slip yoke, trans mount & front brakes, bearings & front U-Joints.
It's definitely better. The brakes were definitely dragging, but every fix reveals a new problem on this old Jeep.
My rear drums are warped & it's obvious w/new front hardware.
I still have the harmonics ("wubwubwubwub" sound).
It appears to be my Chrysler 8.25 rear diff doing all that humming.
I got one in better condition for $25 from a guy who swapped in a Dana 44. It has drums in better shape than mine, too.
Gonna install hopefully before I drive 1600 miles next week.
Good luck with yer truck!

 

Jumping in on an old thread, did any of you guys figure out a solution? Replaced the drive shaft u-joints and went on the highway and i'm still getting the harmonic vibration. I bought an XJ two weeks ago, slowly picking at it...replaced motor mounts and tranny mount (they were practically non-existant, one side the bracket bolts were sheared right off the block) and i'm wondering if putting everything back in its correct position is causing a wear where things may have been comfortable where they previously sat. The rear end took a whole bottle of gear oil too, starting to think someone wasn't too nice to this ol' jeep.