Go to the mission THAT IS BEFORE THE LAST MISSION. Choose a fast car before you go to see Goodman, drive infront of him until you reach the desert- still follow the road until u reach the point where you need to turn left, but dont turn! when you see the army truck GET in the truck (the truck will be on the road) THEN abort mission, but you MUST be in the truck before you abort the mission. Don't chase Goodman, Just abort mission. After you abort mission, the truck is all yours!