I PUT VEHICLE IN PARK AND GOT OUT TO CLOSE A GATE ON MY FARM PROPERTY. WHEN I REALIZED THE VEHICLE STARTED ROLLING BACKWARDS, I TRIED TO GET BACK IN TO STOP IT. THE DOOR KNOCKED ME DOWN AND PICK UP ALMOST RAN OVER ME. I DID A BACKWARDS SUMMERSALT TO GET OUT OF THE WAY OF ROLLING PICKUP. WHEN I WAS FINALLY ABLE TO GET UP AND GET TO PICK UP, IT HAD SLOWED DUE TO THE JACK KNIFING OF THE BALE BUGGY I WAS TOWING. PLASTIC AND RUBBER IN THE SHIFTER HAD TO BE PUSHED BACK INSIDE SHIFTER CASE BEFORE I COULD PUT BACK IN PARK POSITION. LATER THAT SAME DAY, I PUT IN PARK IN PARKING LOT AND IT JUMPED INTO REVERSE AGAIN. LUCKILY I WAS STILL IN VEHICLE. I BELIEVE THE PROBLEM IS IN THE SHIFTER CONSOLE BUT ACTUAL PROBLEM IS UNKNOWN. THIS HAS HAPPENED A TOTAL OF 4 TIMES. TWICE I WAS OUT OF VEHICLE. TWICE I WAS STILL IN DRIVERS SEAT. ONLY ONE TIME DID THIS PROBLEM RESULT IN INJURY.