I've been searching various threads and haven't found what I'm looking for yet, thought I'd go ahead and post in the meantime.

Looking to buy a second XJ, it's a 2001, 4.0 2wd auto, 152k on it. Sellers ad said it "has an intermittent vibration sometimes in park and also while driving, might be the motor mounts".

Took a good look at it, having just changed the motor mounts in my current XJ this past Sunday, I can see that these are fine. Took it for a short test drive, initially no abnormal sounds or vibrations, got up to about 40 mph. Pulled over to turn around after about four miles and came to a stop, still in Drive, and then the vibration started. Definitely not an exhaust type rattle, this sounded and felt like something rotating was out of balance. Shifted to park and it persisted. Lightly tapped the throttle and it went away (maybe coincidence?), put back in drive and returned to where I started with no further issues. Other than this unknown the Jeep is well worth the asking price, just don't know how big of a leap I want to make.

Any ideas?

Edit: Total duration of the vibration was probably less than 10 seconds. Definitely not in the rear of the car, and didn't seem like all the way up front, somewhere from the back of the engine to the back of the console?

We're the rpms dropping while it was doing it? Could be an idle issue with the engine.

Not that I noticed. The vibration was way more than a change in rpm would cause though.

My though was if the revs were low and it could be dieseling.

look at the harmonic ballancer

Misfire code, does it overheat?

Hadn't thought of that but that's the kind of shaking that's going on.

How do you inspect that?

No codes that I know of, didn't notice it running hot but it was a 10 minute test drive.

The rubber separating the two halves begins to degrade

Thanks, I've asked for a picture from the seller as the vehicle is about an hour away. We'll see what that shows!

Supposed to get pics today. Seller is now saying that vibration only happens when AC is on?!? I can't imagine that would be enough to do this, it was a heavy shaking?

Is it worth a new engine?
Just a thought, you may want to have this vehicle inspected by a competent mechanic before this goes any further. This is like trying to perform surgery over the phone.

Asking $2500. Other than the vibration issue it's a nice clean Classic (with dog hair) and a sagging headliner. New tires on it, no rust or damage. Was just trying to find areas to look at for the vibration. Yes, I could swap a motor in but I really don't have the time right now. Nor do I think that's the issue.