I'd like to get the feedback of others on the board. I have a 98 Mustang GT with 4 different tires with each tire at different tread levels, so maybe this is the route cause. Maybe getting 4 new MATCHING tires and a good balance would correct it? Anyway, the car always moves smoothly until I get onto the highway at hit speeds between 65-70mph when the whole car starts to shake. If I exceed speeds to 75, 80, 85mph the car continues to shake but nearly as bad. If I drive anywhere under 60mph the car rides as smooth as a babys butt. Thoughts?