The pilot stated that after takeoff from the heliport, 'He felt a low frequency vibration.' He elected to return to the heliport to determine the cause of the vibration. After the pilot landed, he had the collective positioned to full down when the helicopter began a violent forward and aft gyration. The main rotors struck the ground. Examination of the landing skids revealed that the left (4H) shock strut assembly had separated from the landing skid due to severe exfoiliation corrosion at both attaching lugs. All three remaining struts were attached.