" Usually higher speed vibration are related to a wheel balance problem. However, I would check the wear on the tires first. If there is a choppy wear to them, then you may be getting a little roughness felt from them in addition to a balance problem.
Since there is also a vibration intermittantly when braking, then you need to determine when it is happening. My guess it is happening when braking at higher speeds such as when getting off a thruway. The more difficult part of this is determining if it is from the front or rear. You " may" be able to use the e-brake to help determine this. At long as the e-brake works, you can hold out the release so it doesn't lock on, and use the e-brake pedal to brake with. If no vibration, then most likely it is the front rotors.