Vehicle was parked on a hill. At the time the vehicle was pulling a light weight trailer (empty). The vehicle was in park when the engine was shutoff. Owner went inside the house, came back out 5 minutes later to find the vehicle had rolled unattended backwards causing the trailer to jack knife. The result was damage to the truck and trailer. This was very lucky considering another vehicle could have come over the hill and hit the truck and trailer and people could have been injured. This is the third instance of this truck rolling unattended. I have contacted Ford, written complaints. A recall was released that fixes a known transmission issue with near VIN numbers but it does not effect this vehicle. Please help.