shaking when braking at 50mph

It sounds like your brake rotors are out of round. This is usually caused by excessive heat braking, replacing the brake pads without resurfacing the rotors or rotors that are too thin and cannot dissipate the heat built up during braking. Have the rotors measured for thickness and resurface them if possible. Replace the brake pads if they are worn, or use some sandpaper to scuff up the surface on existing good pads to create a good mating friction surface to the fresh rotors upon reassembly.