The most important question is: Is the vibration in the steering wheel, or in the body (the seat of your pants)? If it's in the wheel look to the front end or front tires, if it's in the body look to the rear end (including rear tires) or driveline. Can you drive through it, or does it get progressively worse as you go higher speeds?
It could be anything from driveline vibration, unbalanced tires, separated tire, etc., etc.
