Chock the trailer tires real well. Back the pickup under it. Stack some boards, at least 2x10's in a cris cross pattern and ease the hitch down on the stack with the landing gear. Make sure everything is where nothing can move.
Take some measurements, raise it back up, and go from there. That way you don't alter/change the truck if you need to visit the dealer for a little heart-to-heart about getting the truck out of nosebleed territory.
My 2 cents. As someone else said until you put the weight on the truck you don't know what you need.
