Injured backed 1 ton truck up on a 1 ft high bladed berm on level ground to replace a U-joint. EE put the truck in park, set the brake, and chocked the front wheels. With the drive shaft removed, only the front brake and chocks were left to hold the truck on the berm incline and they slid 2.5 foot forward on the muddy ground causing shoulder bruising by the rear dual wheels.