[consultIII]

does it shake only on accelleration or even when cruising at 60?

possible engine mount.

Are the tires a quality brand tire; as I have had cheap new tires balance ok but they had excess radial tire variation. As the tires are now 3 weeks old, the tire plies have relaxed, you could now have them road forced balanced to confirm. Typically this condition would worsen with road speed.