I bet it's an ex-ambulance or RV.
You can find pretty good deals on E350 diesels in that configuration. Yank the box off, stick a flatbed on, and cap the walk through in the cab, got a pretty capable truck.
Only downside is that very few of them are 4x4. Ambulances get the crap driven out of them, but also get maintained meticulously.
I had a friend with a Toyota Hilux dually flatbed. Had the 22RE in it, I'm pretty sure it started life as an RV. Handy little truck, he sold it a few years ago, saw it stacked to max height with appliances in the city a couple years ago.
Had a neighbor in Minnesota that was a paranoid schizophrenic. Good guy on his meds, not so much when he wasn't. Did some time for murder. He used to take cars and cut the top off so he could fill them to the brim with left over produce, bread, whatever he could find to feed the hogs. Got locked up in 2005 for carrying a gun, not sure if he's still there or passed away. The guy was sort of a mad max genius with equipment, walking around his farm seeing his creations was pretty entertaining.