I first noticed a slight driveline-speed vibration about three years ago, and
it's gotten more noticeable over time.

My Chevy experience says U-joint, U-joint, U-joint.

I read all the comments about the $350 u-joints, mystical balance issues, etc.
So I'm hesitant to just throw on a new shaft.

- Car is a 126 ('91 300SE) with 160k miles.
- Vibration seems definitely propshaft speed, and ONLY occurs under
two conditions:
1. Acceleration at 15-18mph. (1800rpm in 2nd)
2. Climbing a grade in high gear at 65-75mph (3500rpm in 4th)
Otherwise, the car is smooth as silk.

1. Checked out the exhaust carefully - it is definitely not the Bad Guy.
2. Replaced motor mounts, and tranny and dif mounts.
3. Pulled the shaft and replaced the frt flex disc and the center bearing/support. The rear disc looked good. Marked everything and put it back in same position.
4. Tried "clocking" the front shaft connection to another position - no help.
4. Wiggled the half-shafts and watched them turn with the motor idling on the lift. They move smoothly and don't bind, the boots are good.
5. I did find two possible issues - could these be the culprits.
a. YES INDEEDY, the u-joint on rear shaft has a bit of a detent on-
center, but I cannot feel any click or looseness - most of the posts
here say this should probably not be a problem.
b. Just a bit of play in the rt rear wheel bearing - you can detect a
slight click when yanking the tire back and forth (if it were a front
bearing, I'd turn the nut maybe 1/16th turn to reduce it).
I actually put the dial indicator on it as best I could and it
measured in-spec. but the left bearing has no discernable play at
all.

Could the u-joint or the wheel bearing be contributing?
Both of these are expensive, and just don't appear to be bad enough to
cause the problem.

Thanks,
Guru

Due to the nature of the center joint, there is very little rotation of the joint. For this reason, they develop wear and binding in a very specific spot. Often, you can change the movement of this spot ever so slightly and the vibration will go away.

You can do this by loosening both bolts that hold the carrier bearing mount in place and then remove the bolt that goes in at an angle. Put a washer between the carrier bearing mount and the body and put that bolt back in through the mount and the washer.

This will move the rotation off of the original position just enough to eliminate the vibration in most cases.

Good luck with it,

Thanks, Larry. Yes, I saw your earlier post on that.
Thinking more about the effect of moving the carrier bearing slightly,
this would either increase or decrease the rotational movement of the
u-joint spider minutely. Initially i couldn't justify to myself how this
small change in the rotation could effect vibration of the shaft.
However, maybe what is really happening is the harmonics of the whole shaft assembly are affected by moving the bearing slightly, and the orbital wobble is reduced??????

Hey, whatever. It's gotta be worth a try. Not so easy for me to do, I may have to pull the exhaust and the big shield on my car, but i'll give it a try soon.

Thasnks again,
DG

larrys right on track with this.there is a specific set of angles of inclination for your tranny to driveshaft to ujoint to driveshaft to differential.these specs can be found in your shop manual.adjustment is acheived with washers or spacers at the various mounting points.this can be a trial and error process or the various angles can be measured by protractor