98 XJ 4.0 AT - Driveline Vibration
 

185,335 total miles - While driving home there came a sudden onset of driveline vibration that shuddered the car. I was accelerating (smoothly and evenly) from a traffic light when it started. I pulled over into a parking lot and drove around slowly. Then I stopped and put the car in park and rev'd the engine. It sounded normal. Oil pressure at 2000 RPM is 40 PSI, and idle oil pressure is about 25 PSI. I had just changed the oil about 20 minutes before that and was taking the truck for a spin. The oil that came out of the engine was black and just about 5 quarts total. Having gotten this 98 from an acquaintance, I have zero history on its maintenance.

Before yesterday I could barely hear anything of the car because the previous owner had mounted some big all-terrain tires on the truck to make it look like a 4wd. Right. I know. So, yesterday afternoon I put the same 215-size tires on it I have on my 96 XJ 2wd. Driving away from the shop I thought the tires were noisier than the tires on my 96 but with nothing else crossing my mind about why I continued road-testing the truck. The noise of the new "tires" was probably leading up to the sudden and excessive vibration noted above.

I limped home and put it in my garage. I got underneath it to look at the driveline. The driveshaft u-joints are tight, the differential oil is at the proper level and clean, but I can wiggle the transmission tail shaft probably 1/8" either side of centerline. I wouldn't think that's a normal amount of play.

Could the loose transmission tail shaft (yoke) be the source of the vibration? Thanks.

Did you check the motor mounts?

Yes I did.

What I did right after posting this thread in a procedural progression -

1. Pulled the driveshaft off. Four bolts.
2. Blocked the transmission up with a floor jack and a 2X4 to take pressure off the transmission tail mount.
3. Removed the cross member supporting the tail mount. That's one bolt and nut per cross-member side and four nuts on the rubber isolator.
4. Removed the bolts holding the exhaust pipe isolator and rotated it out of the way to gain access to the tail mount rubber isolator.
4. Removed the tail mount rubber isolator. Two bolts.
5. Removed the speedo transmitter and additional sensor (don't know which one it is) from the transmission extension housing.
6. Removed the 7 bolts from the extension housing and smacked it with a couple 2X4's to break it loose from its gasket.
7. Removed the transmission fluid lubrication tube from inside the extension housing.
8. Degreased and then plastic bead blasted the extension housing exterior surfaces followed by a glass bead blasting.
9. Alodined the blasted surfaces.
10. Cleaned for inspection, I inspected the interior of the housing.

(Total time to accomplish 1 thru 6 was about 30 minutes, after running through my vocabulary of expletives psyching myself up for the job).

Findings: The rear transmission oil seal is shot but the leak and the vibration were due to a worn bushing located immediately forward of the seal allowing run-out of the transmission shaft noted in my first post. This bushing keeps the extension shaft of the transmission running true.

I called around and got the whole laundry list of items coming in to O-Reilly's tomorrow. I'm replacing the drive shaft u-joints, the extension housing bushing, seal and flange gasket. I'm searching for a local drive-line facility that can balance the drive shaft while I have it out. I have to bring in the extension housing to ensure I get a proper match-up of the seal and gasket. They have various versions.

BTW, a leaking rear seal on the transmission extension housing is usually caused by a worn out bushing. Replacing the seal will only fix the symptom, not the cause. This I found out today while researching my vibration problem.

Wow, glad you figured it out