well, this is not your average shaking problem...

car shakes while going 70+ mph but there is nothing in the steering wheel, just the car... that usually means the problem is in the rear side. well I got the wheels balanced several times and it's still there. I suspected an axle bent, got it checked out, but it wasn't... my mechanic says everything is in order and working fine.

I'm out of ideas here, what can it be?

 

check for missing weights on the driveshaft (front or rear), and have the tires checked for a slipped belt