My DD is an '03 CLK500 with 32,000 miles. The last while it's developed a problem I'm hoping someone might have some thoughts on.

Under light acceleration between 1200 - 1600 rpms (mostly 1st and 2nd gear) there is a nasty vibration type of sound that seems to be coming from somewhere in the rear of the drivetrain. When the drivetrain is under heavier load (eg. going up a ramp), but still under light acceleration and about 1200 rpm there can be a studder for 2-3 seconds (again feeling like it's coming from the somewhere in the rear). Under heavy acceleration the vibration does not occur.

The dealer has checked the complete suspension and can find nothing. They can hear the noise but can't find it. Car shifts fine, so I don't think it's the tranny. It's not a wheel problem as it only occurs under light acceleration, dealer has checked the wheels and they are not out of round and are balanced.

Any thoughts? ... and thanks.

could be any of the universal joints, cv joints if any, or transmission/motor mounts

Thanks for the help, I'm taking it back in again soon for them to have another crack at it and I'll pass along these suggestions.