What causes the steering wheel to shake when I'm not even moving?

After replacing the steering rack in a 1999 Ford windstar, the steering wheel shakes violently when the vehicle is started. The steering wheel is uncontrollable and the vehicle can't be driven. The steering wheel actually moves lock to lock by itself. I've never seen anything like this.

Some Fords and Mercury's have electrically assisted power steering. A glitch can arise in the system that causes the motor to oscillate back and forth like that. The problem needs to be fixed by an authorized ford mechanic.

Try another steering rack

Maybe one of the hydraulic lines is damaged or squished. Did you recently hit something from underneath the car? Like a big rock, brick, or something that may have damaged a hydraulic line that goes to the steering mechanism?

Hydraulic steering assist in your 1999 Windstar. That can only mean the torque sense valves on the top of the rack are sticking. Replacement of the rack is the only solution if I haven't overlooked something. I wonder, though, if you grab the wheel forcefully and resist the movement does it at least try to respond? If so, it is a safe bet the steering rack has to be replaced.

EDIT - one more thought: is the power steering reservoir decently full and has no foam when the engine is running? If the fluid is foaming odd things can happen.

Engine knocking. It needs a mechanic to look at it.