Agg Truck Drivr was going to drive R25 Haul Truck, #14-0339. After checking out the truck, while @ the N. Quarry Shanty @ approx. 7:15 AM, he was stepping back out of the cab, he slipped off and fell 4' landing on both feet. In the process he was holding the right grab rail and injured his rt. arm shoulder requiring medical attention.