my firm belief is that there is a harmonic in the engine trans mount configuration that cant be solved by laymen. there's probably a good reason MB never put 4spds in 300 turbos. my guess is the NA need to operate at higher RPM since they do not have the power to even try to push the car at 1800RPM. as long as you are above 2000 RPM there is no vibe.

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1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)