I can't speak from experience, but looking at how it goes together and having been in at that rear end a few times:
It would take a socket - but not too bad.
Looks like there are two frame pieces that bolt to the tie-down points on the bed of the cargo area. The box bolts to the frame pieces.
The biggest question I would have is if you need to remove the box to yank the frame pieces -- that would entail emptying out the entire box to get at the bolts in the bottom, which would probably double the amount of time it takes to remove this.
It doesn't look like a lot of room there to get under there and back at those bolts on those frame pieces where they bolt to the tie downs. You might be able to fold down the rear seats and get at it from that direction though.
If you do ~not~ have to remove the box first - looks like just 4 bolts and it would slide right out the back. If you ~do~ have to remove the box first... then empty the box, 4 bolts to pop the box off the frame, then 4 bolts to remove the frame pieces.
Given that it's 16GA steel - it probably weighs a good bit, and it's a bit tight getting it in and out - so while you could probably do it solo, I'd imagine those rear quarter panel trim pieces would get pretty dinged up doing it by yourself.
I definitely have a use for a box like that and looking at a used Bronco that already has one. I did see how it's installed and the bolts aren't really a problem and I'd likely convert it to a quick disconnect system. I just want to know if a person with reasonable strength can slide it out the back. I'd disconnect at the bracket/Bronco interface leaving the brackets and the box as one piece for removal.