EMPLOYEE REMOVING RIGHT SIDE PUSH ARM ON A D11 BULLDOZER, STARTED TO PICK UP DOZER BLADE PYSH ARM WITH SERVICE TRUCK CRANE TO LET DOZER BLADE SLIDE FORWARD. CHAIN WAS NOT IN LINE, EMPLOYEE ATTEMPTED TO ADJUST THE CHAIN WHEN LOAD SHIFTED CAUSING THE PUSH ARM TO JUMP OFF TRUNION, EMPLOYEE WAS THROWN BY FORCE OF PUSH ARM INTO THE BUMPER OF HIS SERVICE TRUCK.