I made my own drive on / drive off ATV rack. Actually at one time it was built to carry two up on top one over the cab and one over the bed with a third in the bed. I carry two sport ATV's I made it to fit rather snug but it could be made longer if need be. The way I have it constructed I could make an extension for the back. For the bed I used a set of bi fold ramps with section in the middle I made out of steel and rebar to get the extra length. The ramps are 8ft with the old 3ft sections left over from the over head rack to make them roughly 11ft. I made it to fit just over the bed I didn't want it sticking out into my mirrors, maybe 6 inches wider would have been better but it works for me. The frame stays in the truck year around I take the rest of it out as needed to use the truck it only takes a few minutes. With the extra weight over the bed I have a set of Firestone air bags, on board air compressor with an in cab regulator to regulate pressure as needed.
Maybe not the best pic of the ramps the extensions are folded up under neath. I can unload with them like this.