Be sure to check out your front-end. Tie rods, ball joints...
A vibration through the steering wheel is usually related to your front axle. While a vibration through the seat, or floorboards is usually something from the rear axle. Cross the front wheels and see if that changes the vibration to the right side.
Your best bet (if you don't like the clip-on weights on the outside) is to use tapeweights. It's the best compromise between looks/ride quality.
Vibration diagnosis flow chart 
