speed specific vibration
 

I have a couple of vibrations I would like to figure out.

All the pertinent info about the jeep
1995 cherokee
4.0
AW4 auto
NP231
Tow package
Dana 30 front, dana 35 rear (no limited slip or ABS)
DD wheels 235 75 15, party shoes 31x10.50
new timken bearings up front
3" roughcountry lift

Vibration 1) Occurs at around 35 mph (32-39ish) when under power, but is only noticeable when I have my DD tires on it (I can feel a slight vibration with the party shoes, but not nearly as much). It does not vibrate when applying the brakes, and I cant seem to feel it at WOT (then again, I blow passed the vibration's speed range pretty quick, and the engine vibration might mask it anyway). When I am giving it light to moderate power it vibrates, and it seems to vibrate more when I put my foot into it (except at heavy throttle as stated). It vibrates the steering wheel enough to visibly notice the movement, and also the dash. It is a fairly fast vibration, kinda like driving down washboarded gravel roads or cattle guards. My DD tires have been balanced.

Vibration 2) Occurs around 50 mph (on the speedo). When I first got the jeep, the PO only had the party shoes, and it did it really bad until I had them balanced, then it seemed to go away. After getting my DD wheels, I didnt notice the vibration. When I put the party shoes back on for a camping trip I noticed it again, not as bad as before, but still there. I figured I threw a wheel weight, but now I am feeling it on my DD wheels as well (not as bad as the party shoes, but it is definitely there).

any ideas what it could be?

Also, what are the symptoms of bad u-joints?

Do you have any play near the rear of you drive shaft. Could be pinion seal / bearing in the rear-end.

I can't check right now because the jeep is in the shop getting a new AC comp put in ($50 more to have a shop do it than do it myself, I know when to quit)

Would the play in the rear end potentially cause a muffled clunk/flutter sound when letting off the throttle at low speed (around 5 to 10 mph)?

I just cleaned and filled the rear diff a week ago, and I havn't noticed any leaks around the bearing

Yes, the clunk from the rear will be heard when letting off the throttle. Because you are no longer putting pressure on the gearing....

How long can the rear diff last like this? What all will I have to do to change out the pinion seal/bearing? Or would it just be worth it to get a limited slip? If so, what years and trim levels had the LSD?