I revered my truck up a slight incline in reverse in 4 wheel drive. After I stopped in reverse I removed my vehicle out of 4 wheel drive back into 2 wheel drive. I put the transmission in park and removed my key. I unloaded items from the back of the truck. During this period the truck began to move forward down the incline building up speed as it went. It struck a tree stump and came to rest there. No one was in the vehicle and I confirmed when I got in it was still in the parked position. It appears the vehicle never engaged the transmission to the parked position.