You are in park. One wheel has to be on the ground to lock the other one when in park. Otherwise the diff will allow the other wheel to spin in the opposite direction. You could use a long bar in the wheel studs of the other wheel that hits ground.
Also if needed remember the spare tire has a hole in the middle so you can still reach the axle nut with the tire mounted. Tire bolted on, and a long bar through one of the holes in the spare's rim to keep it from spinning.
[This message has been edited by phonedawgz (edited 08-24-2014).]