I would suggest it is very likely that the problem is what MB DOC posted about in the beginning. The touque converter lockup clutch will lockup on that car at about 35 mph hour in gear 5 and 32 mph in gear 4. The lockup clutch has a computer controlled slip at those speeds to isolate any drivetrain vibrations that may result. The slip under no acceleration should be equivalent to about 60 rpm at those speeds. If you are not getting the proper amount of computer controlled slip out of the lockup clutch and it is engaged then this may be the problem. The proper amount of slip should amount to just barely a needle's width over 1500 rpm at 40 mph at steady crusing speed with a constant throttle position and with the lockup clutch engaged. No slip and it would be 1500rpm at 42 mph. This small difference can cause a vibration.

Has the transmission controller been replaced? I know of at least one instance in which a 97 E320 without an updated controller would do this, new controller fixed the problem. At those speeds, is the torque converter lockup clutch engaged? If so, will inducing it to release eliminte the vibration?

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record