RAM 1500 WAS PARKED STATIONARY ON THE HILL IN FRONT YARD WHILE UNLOADING SIX BAGS OF MULCH. THE TRUCK WAS IN PARK,KEYS IN POCKET, ENGAGED IN FOUR WHEEL DRIVE, AND PARKING BRAKE APPLIED. AFTER ABOUT 10 MINUTES OF UNLOADING, THE TRUCK SKIDDED DOWN THE HILL AND ENDED UP IN A DITCH. WHEN TRYING TO REMOVE THE TRUCK FROM THE THE DITCH, UPON IGNITION THE RAM WARNED ME TO DISENGAGE THE PARKING BRAKE BEFORE MOVING THE VEHICLE. SOMETHING LET LOOSE TO CAUSE THIS RAM TO "RUN AWAY." I HAVE CONTACTED MY LOCAL INSURANCE AGENT AND FILED A CLAIM WITH THEM AND RAM THE CLAIM NUMBER FROM RAM'S SPECIAL INVESTIGATION TEAM IS 31851709. MY TRUCK WAS DAMAGED AND CURRENTLY RESIDING AT A COLLISION CENTER FOR REPAIRS. I WILL NOT GET BACK INTO THIS UNSAFE TRUCK UNTIL AN ANSWER IS FOUND OF WHY OF IT LET LOOSE ON A GRADE. MY NINE YEAR OLD SON WAS WITH ME IN THE FRONT YARD. THIS IS CAUSING ME NOT TO PUT MY FAMILY BACK INTO THIS TRUCK, UNTIL A FIX IS FOUND.