Why is there tremendous vibration when I apply the brakes over 55 MPH? on 2000 Buick LeSabre

Vibration diminishes greatly once below 50 MPH.

Asked by for the 2000 Buick LeSabre
your brake rotors are probably warped and the speed (increased ossilations)is setting up the vibration. if the rotors have enough material left they could be machined but i wouldn't really recommend that as much as replacing them with quality replacements and making sure that the mounting surfaces on your hub is free of rust/corrosion as this will interfere with the run out of the rotor. if you have them turned there is less material there and they won't act as a good heat sink and will warp easier. also have your lug nuts torqued to proper specs in a star pattern as the proper clamp load on the wheel which is holding the rotor on will be applied evenly as not to put an uneven load on the new parts-hope this helps