its either the tranny like Aaron mentioned or the front tires. have you put any fix-a-flat in recently? try rotating and balaning them to see if it fixes the problem or if the vibration moves from the steering wheel/peddles to the seat.
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1996 with 254k miles, L32 4" FWI -> ported N* -> Ported Gen V w/3.0" Pulley, Stage 3 Phenolic I/C, ZZP FMHE, 1.84 RR, Headers and 3" pipe to mufflers, F-body brakes, and lowered on Eibachs. -RIP
AMG C400 White on black. Stage 2 w/E30 - 11.9@117 -daily