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