I have a 97 Grand Cherokee, AWD 5.2L 318. between about 25-60 mph I can hear this strange humming noise coming from the mid-rear end. to put it into perspective, it sounds similar to a bad wheel bearing, which is what I initially thought. but it only makes the noise when accelerating. It also vibrates slightly when put into gear. U joints and motor mounts appear to be tight and in tact...Im stumped, please help

 

LOL I have the same problem on my 94 zj...@ about same speed but i hear it and feel it driver side front ...havent figured it out ...drives me ape ..the only thing i havent checked is the driveshaft balance..i'v landed on it so maybe.

 

I would check the u-joints again. Even though they feel tight they can still be bad! Have you have ever had the gears replaced/repaired in the rear end???

 

Same thing on my 94 5.2. There is a tsb on it (technical service bullieten.) Might not be the same issue but....

Just FYI - I finally completed the Drivetrain vibration @ 2400RPM TSB (TSB 09-06-97) on my '93 ZJ 5.2. My ZJ had some horrible vibrations coming into the cab when it was under load and at approx 2100-2500 rpm. It also sounded like I had a large exhaust leak. Apparently this TSB applies to '93-'95 5.2 V8 ZJ's. Here's some info which may be useful for some of you on here...

I did this TSB in two phases (due to my college student budget). The first phase consisted of replacing the engine mounts. This fixed about 10% of the engine vibrations my ZJ had. This phase took me approx. 2 hours to complete with normal hand tools and a lot of swearing . Part numbers and prices for the engine mounts (from my local stealership):

Left Mount 52058504 - $85.02
Right Mount 52058505 - $90.42

Phase 2 consisted of replacing the transmission mount, crossmember, and transmission mount bracket with newer and beefier components. This is what I installed today, and it took me about an hour to complete in my driveway. My ZJ feels like new now!! I could honestly say that this took care of the remaining engine vibrations that my ZJ had. I am definitely a happy camper now! Parts and prices:

Crossmember 52058765 - $111.60
Trans Bracket 52058784 - $40.02
Trans Mount 52058488 - $0.00 (not sure why I wasn't charged for this...)
Bolt 6101670 - $8.16 (4 required - for trans mount to crossmember)
Bolt 6033358 - $5.40 (1 required - long bolt thru trans mount)
Bolt 6504553 - $4.74 (1 required - additional bolt req'd for trans bracket)

There is an additional part called a 'stiffening kit' (part# 4883334) outlined in the TSB, it costs about $200. It appears to be some simple bars that are bolted to the engine block and transmission bellhousing. I chose not to install these due to the ridiculous cost for these simple 'bars'.

Also, there were some pre-drilled holes on the transmission that needed to be tapped in order to mount the new transmission bracket to the tranny. The new bracket has 4 bolt holes vs. 3 bolt holes on the old bracket. You will need a 7/16" X 14 NC tap if your transmission has not already been tapped.

There is the tsb right up on another site.