When I place my vehicle in park on my driveway, it rolled backwards and collided with the vehicle behind it. when this event occurred, the truck was in park for over an hour before rolling backwards. This is not the first time that it has done this and when I saw it happen the first time, it was only for a distance of two feet and all the wheels were turning when it rolled backwards. The truck was only in 2 wheel drive mode and 4 wheel drive automatic mode. We are now trying to leave it in 4 High wheel drive when parked. Our driveway is on an incline and we do live in Mn. Our driveway was clear of snow and ice when this occurred and the temperature was 40 degrees. We are not allowed to park our vehicles on the street overnight but have asked for a special permit given the damage to the vehicle parked behind the truck that was hit when the truck rolled backwards. This was over 4 feet when the truck rolled and the vehicle that was hit stopped the truck from rolling further. We confirmed the truck was in park when this incident occurred.