If your car only shakes at high speeds then it could be a bent wheel or your wheels need alignment.
Another cause is your wheel weights fell off causing your tires to become unbalanced or your tires are wearing out unevenly due to the misalignment and causing the tire to become out of round which can cause the car to shake at high speeds.
Another cause could be your tires or tire is getting a bulge or other bad spot in it so inspect your tires.
Other causes of your car shaking at high speeds is an out of round or warped brake rotor, or brake caliper that sticks on.