I get slight vibrations around 45 mph, but if I go over 70 mph, the car acts as if it will shake itself completely apart!

This has gotten worse over the years despite 4 sets of tires, a 4-wheel alignment performed during tire replacements, and several wheel balances.

I actually avoid highway drives in this car due to the excessive vibrations...tragic, since the engine runs well and seems to enjoy flat-out motoring!

I don't think it's engine speed related since the vibration seems to manifest itself at different road speeds while in 4th gear. My indie checked the motor mounts last summer and deemed them healthy.

Don't know the condition of the flex discs or other drivetrain components. Where should I start looking and are there telltale visual clues of certain worn parts?

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car