clearify your vibrations. where do you feel it like front, rear, middle like tranny tunnel, does it vibrate through the steering wheel, come on at a certain speed and go away. this helps. what brands tires do you have? I had set of kumhos once that wouldn't balance for nothing, they did alright on the machine but on the highway the would shake, the tire shop was confused wasn't the wheels so they tried new set of tires.. shake gone. a driveshaft can cause vibrations, some have smal weights on them if they where balanced from the factory, also if they dented by hitting something this will cause and out of round condition and make them vibrate but this really unusual for a mustang.
describe your vibration better and others will chime soon enough.