I don't know how other trucks work, but Ford tailgates can easily be lifted off when you open them a few inches. A solution that I use, while not foolproof, will discourage a thief to where hopefully he will decide to move on to greener pastures.
Get a radiator hose clamp that uses a screwdriver or socket to tighten and is large enough to go around the metal tube on the passenger side that attaches the tailgate to the truck. Put the clamp around the tube and tighten it. The slot that allows the tailgate to be removed is now covered thus preventing quick and easy theft.
