If I understand right you are trying to remove the nut on the power steering pump shaft in front of the pulley. Is the pump on the truck or off? It is often best to loosen something like this while it is on the truck and the belt is good and tight. With any luck the belt will keep the pulley from turning and permit you to loosen the nut. Second thought is an impact gun. An impact will often remove nuts or bolts that you can't easily get otherwise. If it is really tight you may need a good heavy duty impact gun. The cheap ones will not have enough torque to get it off.