Depending on the year, the rear cross sill may be spot welded to the bed sides next to the tail gate. If you have a grinder handy they are easy to break, but be prepared.
If it was put on a trailer frame you probably don't know how it got attached to the frame, it may not be the same as original, and it may have some welds or hidden damage you don't want to deal with. People tend to abuse trailers.
You can buy a very good repop bed from MAR-K so I wouldn't spend too much on a tired old one unless it is in A-1 shape.
