Have you checked your UJ's, it sounds as if something is loose? if it is speed specific there is a chance it could be wheel balancing. If I recall correctly, the front wheels is represented with a vibration through the steering wheel and the rear wheels you feel a vibration in the general vehicle, not necessarily just the peddle or peddle box. Are you able to drive through it, i.e. say if you get to 65-70mph does the vibration dampen?