The 7' bed will be a little harder to come by than the 6', might be easier/more cost effective to find a 2wd being parted out and buy the entire rig.
If a 6' bed will work, here is one with the oem frame.
http://seattle.craigslist.org/tac/pts/5198179990.htmlWhen you set the trailer frame up for proper towing characteristics, the 6' bed tongue to center of rear rim is almost the same as the '7, bed, but gives you more storage room, bed-to-tongue than the 7', and is about 8" shorter rear to center of rim. This is one that I put together(still in the works) as a dual purpose trailer.
![](http://vvcap.net/db/dZ9LBSQammmQx5ba37Qo.png)