Mark, thanks for the new flatbed/tray link.
A rear trundle drawer is not hard to add if the company doesn't provide it. You just add extra C-channels or I beams to allow for more clearance (before the bed is installed). Then add the drawer slides etc. I saw how they did it yesterday when I checked out the Home Depot aluminum flatbeds/trays in Boulder. (There was no manufacturer tags on these but someone on another thread id'd them as being made by Tafco / Scott, I believe.) In any case, while the trundle drawers are handy they increase the height of the bed and your center of gravity.