If there is play in the steering wheel, it could be the steering gear box. This is not a part you mention having looked at and can cause vibration during what is often a slow failure. Vibration might even be exclusive to low speeds.
See if you have an inch or so of play in the steering. By this I mean, when you try to turn, you have to move the steering wheel an inch or so to turn either left or right before the car actually begins to turn.