Vibrations!!!!!!1

hello guys i have astro 96 cargo van an d everytime i start turning the steering wheel i get a heavy vibration. what is the cause of this

Your vehicle is equipped with a hydraboost system. The pump supplies hydraulic fluid pressure for the power steering and the power brakes. Applying the brakes firmly while the engine is idling MAY produce the same vibration as turning the steering wheel.
Vibration could be caused by:
a) A worn or slipping serpentine belt. Check the belt tensioner and belt.
b) Low and/or aerated fluid. Check fluid condition and level.
c) A restricted hydraulic line or,
d) An internal malfunction with either the pump or the accumulator/valve system. Have the system checked by a qualified technician.

Does this vibration occur sitting still or while underway?