This is off topic but what you guys are describing happened more than once. There was a guy on here that ended up with wrecked truck, tractor, trailer, plus a couple broken bones. He was loading in a gravel drive with a slight incline. As the tractor went on the trailer it lightened the rear of the truck and off it went. He got scared and jumped, shortly after the truck crashed into a tree sending the tractor that was still in gear into the back of the truck.
This was not a big tractor. Something like a 25Hp Kubota and if I remember right it was a large 1 ton dually that took off on him. He said if he would have been thinking the only way to stop it would have been to hurry the tractor up on the trailer to put weigh on the tongue and maybe make the rear wheels stop it. Maybe not. Either way once its sliding its hard to stop. I see it 5 or more times a year at the lake. Guys with a big truck will pull it into the lake with a 3,000# bass boat. They always say the same thing, I had it in park with the brake set. I never ever launch a boat with out the truck in 4 high. This way when you put it in park you are locking up the front and rear axle.
Chris