I'm new to the site, so if I made a mistake dredging up this old thread, I apologize.
I just got a used '05 XLT last week and I've exactly the same problem that the OP had. I'm missing that rear cargo floor panel (or whatever it's called, the little "trap door" that forms the floor of the cargo area). I've been poking about the interwebs all day trying to find a replacement part with no luck. That's how I found this site. And I tried the link posted above, but I don't think they had the part I'm looking for (although there were plenty of other bits and pieces that caught my eye, so thanks for that).
Normally I wouldn't be too concerned with this kind of thing, but this is the first car I've owned that wasn't a barely mobile pile of rust and I'd like to make it last. Thanks in advance for any help you may have!