Check out www.dakota-durango.com and ask/see what they can tell you in their forum.... Looks like a dealer/custom add-on to me. I know they did a few stepside Dakotas back then, but I do not recall a sort of dually look bed....
I tried that place when I first got my quad cab Dak. Most people there seem to be the biggest bunch of self rightous douche bags I have ever met. They seem to like to gang up on the new guy and belittle them, and yes, I was vitum as well.
I even got PM's stating, "Dont F%&# with the boys"
I figure that asking a simple question would be OK, but all I got was bullshit. Theres a couple of good fella there, not many.