I had to get some 16" boards a few years ago for my barn... I loaded them in the bed w/ the tailgate dropped, ran a strap around the tailgate and over the boards so they were strapped in the middle, and bought four 60lb bags of gravel and piled them on top of the boards at the front of the bed. It was a pretty redneck rig but I'm only across town from the lumber yard. Did it on a lazy Sunday with no traffic.
Probably not a legit solution if you have any great distance to cover.
Another thought, if this isn't going to be a recurring need you can probably just rent a trailer for half of what that tailgate contraption costs.