DRIVER WAS HAULING ROCK FROM THE MINE TO THE RAILROAD LOADOUT FACILITY. WHILE EXITING THE TRUCK TO OPEN THE TRAILER DUMP GATES THE DRIVER SLIPPED ON THE TRUCK STEPS CAUSING HER BODY WEIGHT TO SHIFT TO HER LEFT ARM. HER LEFT ARM WAS HANGING ONTO THE DOOR HANDLE. THE WEIGHT SHIFT SPRAINED THE DRIVER'S LEFT SHOULDER.