My 07 shakes steering wheel shakes when in gear sometimes too. A good way to check to see if it is a motor mount is to turn all the accessories off and decrease the load on the engine, if the vibration decreases it is a good indication that your motor mounts are bad or going bad. I really dont see how tires could effect this at all lol. Plus you wont have vibration when your stopped if your rotors are warped, just when stopping.