Vibration in rear end?

first off i have a 93 cherokee stock with 86K mi it has a 4.0 HO a Aw4 trans and a 231 transfer with a Crysler 8.25 rear end

I have a really bad vibration comming from the rear end, it starts at about 2k rpm regardless of the speed, it gets really bad when going up hill. I think its from the rear-end. It does however seem to calm down if i go about 60-60 mph. it is getting worse as i drive it, so i was thinkin it was my bearings in my rear end. so what do you guys think?

am i smoking something and its the transfer case??

should i rebuild the rear-end?

is there any ideas to maybe isolate the problem and do some troubleshooting?

thanks for the help (sorry about the long post)
Nate

I think the first thing to do is really to try to isolate this. What happens when you're coasting out of gear? If you're coasting, what happens if you rev the engine? Does it change if you run up to high speed in third gear instead of fourth? Does it seem to change when you accelerate or back off on the gas?

If this vibration is truly keyed to engine RPM rather than speed, I'd look at engine mounts, transmission mounts, and the exhaust system first. Get under the vehicle and yank on things.

You don't say whether or not you've inspected the driveshaft universal joints. Even if they don't feel loose, it might be time to take the joint off at the differential end and check for binding or dry bearing cups. If you undo the straps on the rear, you can pull the shaft out of the transfer case and check the front joint for smoothness too. The joints should move freely with no play at all. Even a little problem here will be amplified under power.

While you're at it, check the shock mounts. The bushings on the rear shocks will often fail long before the shocks themselves, and this can cause nasty vibration, especially if one of the wheels is a little off balance.

86 K is very young for differential trouble. Not that it couldn't happen, but I'd look elsewhere first.

it happens only under acelleration and under hard acelleration, and it only happens between 2k-3k rpm no matter what speed. the u joints are brand new.. about 4 months ago i broke the rear u-joint and replaced both on the rear driveshaft and had the driveshaft ballanced.

I will look at shock mounts (they are stock shocks) and also ill check motor mounts.

P.S. if i rev the motor in netural it does not vibrate.

Did you replace the U-joints yourself? If you had the shop do it, they may ahve forgetting to lub them up. Stranger things have happen, and these are reasons that I don't trust shop machanics, unless they don't mind me watching them work so that I can "learn about what they are doing". I had a friend a month ago who had to get his transmission replaced (I'm in the Navy and he's on the ship with my and his wife took their truck in). They replaced the transmission and she noticed a metallic sound as she drove it home. So she stopped and got AAA to tow it back to the shop. Turns out they put a new transmission in, but never filled the tranmission fluid.
Just a thought, but they were replaced a month ago, and the sound is getting worse and its louder the faster you go (the more revolutions the driveshaft is turning).
Good luck. By the way, how'd ya keep the mileage so low (yes, guys obviously by not driving it that much)?