CREW TRUCK WAS ASCENDING #1 SLOPE, WITH CREWS IN BACK. EMPLOYEE INJURED WAS HOLDING ONTO ROLLBAR WITH RIGHT HAND. WHEN TRUCK REACHED TOP AND ONTO FLAT SURFACE, TRUCK TRANSMISSION MALFUNCTIONED CAUSING ABRUPT STOP. OTHER EMPLOYEES SLID ON SEAT INTO HIM, PUSHING HIM AGAINST THE ROLLBAR. HE DID NOT SEEK MEDICAL ATTENTION UNTIL 5-10-01.BRUISED RIBS ON RIGHT SIDE.