Yes, mine does the same thing. It's not related to the engine, because it happens in neutral as well, and only above 50mph. It's not present in the steering wheel whatsoever and doesn't feel like front suspension, it's in the seat of your pants and floor. Suppose it could be a tire or wheel but I doubt it. I really feel as if it is a prop shaft issue. I'm going to check out my flex disc as a matter of course and report back.

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1967 230S automatic
Boston, MA

I use to have the same vibration too. It went away after I replaced the itermediate bearing and bushing on the drive shaft.

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Britton McIntyre

68 280 SE coupe 'Hairball'
70 280 SL
71 280 SEL - RIP May 2010