[reallytired]

This sound like it could be problem with brakes, tires, or tie rod. If it vibrates at 45-55 miles per hour I would check the tires or tie rods. Make sure the tires don't have a bad spot anywhere and that the tie rods - different parts of the connecting the wheels are not excessively loose. Also may just need an alignment.
If it seems to be vibrating all the time you might want to check your brakes, rotors, & drums.

I have had this problem before and so these are some of the places you might want to check. If you jack it up you can check most of these yourself.