I have a 2004 f150 heritage. About a month ago it began vibrating badly at hiway speeds. It has a 5spd manual trans. the vibration gets really bad when I put it in overdrive. Thought it was the tires so I switched them, Held for about a week but now its back. Also began hearing a squealing sound that appears to be coming from one of the rear wheels but only at low speeds. Any clues?

 

Check the e-brakes. Heat will cause a vibration like you mentioned and the brake shoes have a tendency to delaminate from the backing plate.