how badly does it shake? if its excessive then that means that the piston firing sequence/timing is off. If its only at a certain speed then, that means either u DO need an alignment or that simply the engine is working at its maximum at this time.
I have a 2000 chevy monte carlo. it shakes or vibrates when I reach a certain rate of speed. I have had the tires checked and balanced and i dont think that i need an alignment.