I just discovered another symptom. With the car sitting in the driveway running in park, if I turn the steering wheel slightly side to side there is a popping sound that seems to be coming, as far as I can tell, from the strut tower on the drivers side. At any rate when I put my hands on the various components down there the tower seems to be the component most transmitting the vibration. With every "pop" you can see the tire slightly shudder. Not sure if this is related but it's definitely something.