EE preparing to air tires on another co. vehicle. F350 was parked on 3-5% grade w/parking brake set & trans. in neutral. Employee was out of his truck starting to air tires, he saw his F350 rolling backwards. He attempted to get in his F350 to stop the rolling, fell under. Front drivers' s side tire rolled over his left leg. Fractured left femur. Minor cut to back of head.