Driving a loaded HT up the ramp, when employee encountered a jeep coming down the ramp. Stopped and began backing up to make room for jeep to pull into muck bay. Employee stated: tried to stop and move forward but continued to go backwards down the ramp. HT glanced off rib and ran into the front of a 2yd LHD coming up the ramp approx. 200 ft. away. HT rolled over on its side.