I own an 88 300 SEL with 164,000 mi. I have noticed a vibration between second and third under moderate to hard accel. Both flex discs have been replaced as well as the center support bearing and rubber support. I change the trans oil every 30,000mi and the oil is clear red with no particulate matter in pan. I have owned this car since 98 and am very familier with it's "personnality. Could the problem be the U joint?

Lots of hits on this in SEARCH.

Try -> 300SEL+vibration

&

Try -> 126+vibration

Lots of possibilities. Good luck

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

The short answer is "yes, it could be." The bearings in the joint can get sticky or wear excessively and cause the driveshaft to bind momentarily or spin unevenly. Which can throw it out of whack. As Mike said, there should be a few postings on this in the archives. You might also try searching for "driveline," "driveshaft," and "u-joint."