AT THE TIME OF THE INCIDENT, EE WAS REMOVING A DRAIN PLUG FROM THE DIFFERENTIAL OF A CAT 793 OFF-HIGHWAY TRUCK. WHILE EE WAS APPLYING FORCE TO THE WRENCH/BREAKER BAR, THE DRAIN PLUG BROKE LOOSE, CAUSING EE TO FALL INTO THE CORNER OF A DRAIN CART. ON 5/8/04 EE CALLED IN AND REPORTED DIFFICULTY BREATHING AND PAIN ALONG HIS RIBS; DIAGNOSIS: CRACKED RIBS.