PARKED ON LEVEL PAVED SHOULDER OF SMALL-TOWN 2-LANE ROAD IN UPSTATE NY. PLACED TRUCK IN PARK, AND VERIFIED TRUCK SHIFTER IN PARK. LEFT TRUCK IDLING IN PARK TO KEEP A/C GOING DURING BRIEF STOP TO WALK TO REAR PASSENGER DOOR TO HAND ITEM FROM REAR FLOOR BACK TO TODDLER. AS I APPROACED REAR DOOR, TRUCK (WITHOUT BEING TOUCHED/BUMPED) ENGAGED AND BEGAN DRIVING AWAY UNDER POWER, NOT ROLLING. CHASED TRUCK AND PARTIALLY JUMPED IN (SHORT LEGS). ATTEMPTED TO PUT TRUCK IN PARK TO STOP IT, BUT SHIFTER IN PARK ALREADY, REFOCUSED ON TRYING TO TURN OFF KEYS. TRUCK HAD ENOUGH TORQUE TO DRIVE ACROSS 2-LANE ROAD, FULLY/EASILY DRIVE OVER CURB, THEN DRIVE DOWN INTO GULLY WITH ME ONLY PARTIALLY INSIDE. WHEN TRUCK GOT STUCK AT BOTTOM AND STALLED, FINISHED CLIMBING INSIDE AND TURNED OFF TRUCK. RE-VERIFIED SHIFTER STILL IN PARK. WITNESS AND HUSBAND ALSO VERIFIED SHIFTER STILL IN PARK. MANAGED TO JUST AVOID HITTING BIG TREE AND SETTING OFF AIRBAG. TODDLER UNINJURED, I DIDN'T APPEAR INJURED AT TIME, BUT LEGS AND KNEES BANGED UP REQUIRING DOCTOR'S VISIT. VEHICLE TOWED OUT OF GULLY AND SUPERFICIALLY REPAIRED ENOUGH TO CAUTIOUSLY DRIVE HOME TO MD FOR FULL REPAIR.