The most common cause of this is an out of balance wheel or wheels. I would check for missing wheel weights and even if you don't notice anything missing, have all the wheels rebalanced. Just a quick question; have you recently rotated the tires front to back and back to front after not having rotated them for an extended period of time? I would also have someone look at all the suspension components. A worn suspension component can cause vibration at speed.
Mike
Mike