Sounds like an out of balanced wheel. The speed specific part of your description confirms that for me. That would be pretty easy and cheap to check at a shop.
If you want to test that theory, swap the front wheel(s) to the rear and see if the front vibration changes at that speed. Wheels in the front out of balance will be more noticeable than the rear end. A good 4 wheel alignment and wheel/tire balance would be my solution.