97 Lincoln mark viii and once you hit about 80mph the whole car shakes you have had a 4 wheel alignment done and a dealership look at the car and no one can find a problem you also had different tires?
Have you checked the wheels to see if one is bent? Bent
driveshaft is another possibility.
I just had to replace the rear sway (no longer available from
lincoln) which was broken in half. Also both sides of front end has
a 1 inch steel rod connecting the bottom of frontend to to frame.
All rubber bushings were worn out. Place car on concrete & have
someone watch the front wheels as u apply brakes, If wheel moves
front to rear on car, these rubber bushing are worn out. Still
avail from Lincoln.