The counter weight and backing pad are two different things. The counter weight is inside the head of the unit and balances out the weight of the backing plate and pad during the orbiting motion. stevet is right, the 7336 already has the 6" counterweight from the factory.
Upgrading to a larger (and therefore heavier) backing plate would actually probably make the vibrating worse.
I don't have a PC yet, but from what I've read, it does vibrate quite a bit, especially at higher speeds. Hopefully someone else can chime in, but what you are feeling might just be normal vibrations.
John