I ran into the same problem. Also I carry a golf cart in the bed and the rear seat footrest goes beyond the jack area. I moved my propane tanks, jack and battery back 12" . Fabricated new brackets and jack mounting plate. The ball mount I used was a curt D900 from etrailer. If I remember correctly the ball mount gave me an extra 7" over a normal length. That alone would probably give you enough.
http://www.etrailer.com/p-D900.html