OWNER USING 2004 DODGE 3500 TRUCK IN FARMING OPERATION. PULLED THRU GATE @20' IN LEVEL AREA, PUT IN PARK, SHUT DOOR WITH ENGINE RUNNING, PROCEEDED TO WALK BACK AND WAS IN PROCESS OF SHUTTING GATE. TRUCK APPARENTLY JUMPED INTO REVERSE, BACKED OVER OWNER AND PROCEEDED TO TRAVEL @500 FT IN REVERSE ACROSS PASTURE AND HIT TREE. REAR DUALS RAN OVER BOTH LEGS AND FRONT WHEEL OVER RIGHT ANKLE. OWNER WAS HOSPITALIZED WITH INJURIES. TRUCK COULD NOT HAVE BEEN IN REVERSE WHEN 72 YR OLD OWNER EXITED TRUCK BECAUSE OF TIME LAPSE BEFORE BACKING UP. OWNER HAS BEEN DRIVING ON FARM FOR OVER 50 YEARS WITH NO SIMILAR ACCIDENT. IT IS COMMON PRACTICE TO PUT IN PARK WHILE OPENING AND SHUTTING GATES.