Is there any play in the steering system. I.e can you move the steering wheel with no reaction at the road wheels when you're stationary.
If there is play, that will allow the wheels to move slightly and set up a vibration which would feed back to the steering wheel.
Check the chassis to steering rack clamp bolts, the steering rack drop plate bolts (and the plate itself to make sure its not cracking) and also the steering rack drop plate to track rod bolts.
Could also be play in the steering rack itself which, in most cases, can be adjusted.
If there is play in the steering, work your way back up the column to check the u joints and splines are all ok.
