Where did you check the oil level?
Have you checked the driveline to see if it is turning the pump? Set screw or key may not be there. If it is OK remove the hose from the cylinder base, install a tee and a pressure guage. Then run the pump at 1000 RPM and see what is indicated. Sholdn't require very much to lift an empty bed, maybe 200-300 psi. Pump should have 1000 psi when deadheaded at a bypass valve. If not then I would replace the pump.
As for the vibration check to make sure the driveshaft is in phase, i.e, all U-joints on the same clock face
