I got a good one for you.
 

So to keep things simple, i have a 2004 chevy blazer 4.3 4wd 4dr with a vibration coming from the rear end only after reaching 50+mph. It has had this vibration for around 3 years or so now. It hasnt gotten any worse but man its annoying. It vibrates just enough that my rear view mirrors shake. Since it has started i have replaced the following:

tires all 4 (balanced by 3 different shops)
rear axle bearings and seals (twice)
both u joints
brakes (ebrake, backing plates, rotors, calipers)
Both sides of split axle

Ive had multiple shops try and diagnose and no one comes up with anything other than "driveline vibration" i have replaced the diff fluid multiple times and even ran it with friction modifier fluids in it which actually seemed to have made it alittle better to where i wouldnt really detect the vibration till about 60mph. I can feel it under my feet, at the gas pedal, ive even felt it at the steering wheel and started to believe it may be coming from the front of the truck so i did alot of servicing and replacing parts there too. Hub & wheel bearings (twice) tie rods, ball joints, all brake components, different sensors i mean you name it.
im the type to do alot of research and i have tried and spent hours upon hourse searching the web for anything or anyone else having this issue and continue to come up emtpy handed. As of today it needs a new alternator as well as an abs module, which im pretty damn sure has nothing to do with it but at this point i think id believe just about anything and im sure any details about the vehicle would help. So hows that for simple?

if anyone out there has any ideas at all i would greatly appreciate it or any questions at all feel free to ask. Ive never had a tougher issue to find or diagnose.

I have a bent rim that does the same thing even though its been balanced.
are the tires new? could be a broken belt in them. Do you feel the vibration in the steering wheel or in your seat

The tires are new. It was making the vibration with the old tires also. Id say i feel it more in the gas pedal than anything. Sometimes i feel it in both steering and seat but more often than not itll be in the seat. Its between 50 & 70. Once i get to about 75-80 it seems to smooth out

Also i have taken each wheel off one by one, replaced it with the spare then taken it for a test and nothings changed so i assume its not a rim

Hmmm, possible driveshaft out of balance.

That is what I was thinking. Take a close inspection to the drive shaft and look for a spot that looks like it might have use to have a weight welded or stuck on, might have slung it off at some point.


Check BOTH drive shafts. Even when you're in 2wd, the front drive shaft still spins while in motion because of normal friction in the transfer case.

As newguy said driveshaft balance is a possibility. Have you checked your rear axle pinion for play? It could be a bad pinion bearing as well

This sounds very similar to a problem I just had. I was sure the vibration was somewhere in the rear of the vehicle. After putting it up on jackstands and running the speed up to 50+ mph I found the problem. The u joint on the front driveshaft was out of position in the yoke. Pulled the four bolts out, slid it over and inside the little tabs where it was supposed to be and it has been vibration free ever since.