[Grant@Tirerack]

Have they checked the wheels for run-out? You may have a bad wheel. If road force didn't find the problem, I doulbt it's the tires. Something else has to be wrong with either the wheels or one of your hubs. The fact that it seems to move front to rear leads me to believe it's a wheel. Have them try a new set of wheels from another car on the lot and see if that helps.