Transfer case or u-joint?

Hey guys (and gals), I posted here about a week ago asking about a new vibration I had in my 96 XJ recently. I first thought maybe it was a worn u-joint and ordered one, I havent received it yet so it hasnt been replaced. Here's the deal. My XJ has a new vibration, it only happens while driving, with my foot on the gas (not when I lay off the gas so I am pretty sure it isnt a wheel/tire balance issue). So, like I said, I thought (hoped maybe) that it was as simple as a worn u-joint (and actually getting under my Jeep and looking at it it definitely needs to be replaced), but now I am thinking it may be the transmission??? So, like I said, it only happens while my foot is on the gas, and in gear (not neutral, or idling). Here are my Jeep's specs: 1996, 4.0L auto, 4.5" lift, 32 inch tires on D30/C8.25 with stock 3:55's (yes, I want to regear but I want to upgrade to D44's first). So, do you think that it could be the transmission with my stock gears and 32's? I drive it pretty hard, and again, it only happens while in drive...I dont know! Any mechanics out there have any ideas? I would rather know before I spend the money on a SYE/drive shaft (oh yeah, I dont have one but I have been running this lift/tire combo for about 3 years with NO vibrations) THANKS!!!!!:cheers:

check tcase mounts abd engine mts.

I have the same noise. A squeaking when I hit the gas a litle or a lot. I went thru my drum brakes, they are not dragging (which was my first guess). I am swapping out the U-Joint right after I type this post so I will let youknow if that helped out. I am running the 4.0 with the Selec-Trac system on a NP242. I just redid al the fluids in the Tranny and Tcase, so we will see.

Yup, that was the problem with my noise. The rear U-Joint of the driveshaft was totally gummed/rusted out. I swapped it with a $15 autozone duralast (they never last) u-joint. Now that I have tracked my problem down I am going to save up for some OX joints that will be good for when I do my SYE conversion later on. I have found that squeaks are only at greasable joints and so forth, if a tranny or TC squeaks dont bother fixing it. Ebay is a great source for parts cheaper than chasing a noise down in a major end item part. I bought a donor 4.0 and tranny for $50 and drove to pick it up for less than 300.
I hope that helps out with where to look for your noise.

By the way, I am not a certified mechanic..... Although I have been working on/modifying and doing total restores on Jeeps for over 20 years.

Thanks, but mine isnt making any noise (squeaking), it is an annoying vibration... I guess I should start by changing the u-joint because that would be the cheapest thing to do and go from there. I was just hoping maybe someone could diagnose the problem online! I'm just worried it will be something more expensive! Like most on here my Jeep is also my daily driver

Sounds exactly like a problem I had about a year ago, it was the u-joint on the t-case side of the rear drive shaft. I pulled the joint and it was FUBARed. I replaced it with a genuine Spicer replacement joint and haven't had the vibes come back yet.

Yeah, I checked my u-joints and the one on the axle side of the driveshaft looks pretty bad. The one on the T-case side looks good. The reason I think it might be something else is because when I pull on the driveshaft it doesnt budge at all...like there is no way it could be where the vibration is coming from, but what do I know? Could the vibration still be from the worn u-joint? My Jeep vibrates pretty bad!

Just pulled the rear drive shaft and all the joints were good and looked over and seen the cracks in the rubber on the transfer case mount. So put in a new mount as well as motor mounts and small vibration went away.

The u-joint will make it vibrate and, sometimes, squeak. My XJ was eating u-joints left and right. Finally, I developed a vibe on mine after a wheelin' outing. I removed the driveline and took it to the local driveline shop to have it checked. They found that the driveline was ever so slightly bent on the t-case side at the weld. The weld had let go a little bit, even though it wasn't visibly evident. They cut off the end and welded it back on straight. They also balanced the driveshaft and put in Spicer u-joints. It has been buttery smooth for about 30,000 miles now. I don't recall the exact cost...somewhere around 160 bucks total? Whatever it was, it was completely worth it. Smooth as glass now!! Good luck.

and btw, it isn't grenading u-joints anymore either...