EMPLOYEE WAS OPERATING A 35-TON HAUL TRUCK. EMPLOYEE STRAINED HIS BACK WHEN THE FRONT OF THE TRUCK WAS LIFTED, THEN DROPPED. THE TRUCK WAS LIFTED BY THE WEIGHT OF THE ROCKS THAT HAD STUCK IN THE TAILGATE WHEN THE EMPLOYEE RAISED THE BED OF THE TRUCK TO DUMP ROCK FROM THE BED INTO A HOPPER. EMPLOYEE WAS RELEASED TO WORK WITH WORK RESTRICTIONS.