I have a 1985 300D, a high-mileage car, but very well maintained. I've developed an unusual problem of rear end vibration at highway speed, but only after crusing for a couple of hours. The problem never appears in around-town driving or during the initial part of a highway trip. This is not the clunking sound of sway bars with wheel movement but a vibration seemingly related to drive train.

Anyone else experience a similar problem?

 

If the problem doesn't occur until after driving for some time then it is likely heat related. The two things I would check are the fluid level in the rear differential and also the brake calipers/rotors. The differential oil may need to be changed or you could have rear brake pistons that are stuck and combined with slightly warped rotors (do you feel a pulsing at all while braking?) they make contact after heating up. The other option is a bad tire, but I would expect that to always vibrate.

 

No pulsing when braking. For the first hour or so of driving at 70 to 80 mph, the ride is smooth as silk. I've gone through a mental list of possible culprits: flex disks, U-joints, drive shaft imbalance, drive shaft bearing. I believe all would exhibit the problem at all times. The differential seems to be the most credible cause factor. I do have a leak at the pinion seal, but not enough to even drip on the garage floor. Could a low diff fluid level cause this symptom?

 

Yes. If the fluid level gets low it allows for more heat buildup.