I was told, and maybe this is just rumor, but supposedly there were no 4x4 trucks built specifically as dual wheel vehicles. They all had those adapters up through '97, even if they rolled off the assembly line like that. I believe there may be some truth to that because I have seen a few 4x4 dually trucks from the those years and then all looked like that.
A lot of google-fu later and your rumor is basically true.
The Dana 60 is the same front axle between a single rear wheel and dual rear wheel 4x4 truck from the hubs inward (at least for the king pin from 87-92ish). They use the same bearings, rotors, and brake setup.
The only difference is the way Ford adapter it for the uses of a dually tire wheel.
I got my spacer off with a jam nut technique that I am not a fan off at all. If the threads fail on the lug stud (which one did on the driver side) you are basically screwed. I had to weld a lug nut to the stud to be able to remove it. (I had been wanting a MIG welder and knew I would need one on this project so I picked up one at Harbor Freight yesterday -->
https://www.harborfreight.com/welding/welders/mig-170-professional-welder-with-120240v-input-64805.html and did flux core to get me through).
So to Ebay I went and found OEM set of hubs, sort of pricey, but they are serviceable and basically last the lifetime of the truck and you don't have to use a jam nut to remove them....
These are getting rare and not many junkyards have anything like that around me, so I bought them.
Also, I can sell my single rear wheel hubs as they are sought after to by the 4x4 wheeling guys, so not too bad of an upgrade, and I can recoup some cost.
I've only made it through the driver's side so far, but I'll work my way through and get the other side off too.
I'm unsure what the back has on it yet, haven't made it that far. It most likely has the same spacer type that was on the front, I'll likely replace it with some newer style spacer as well so I don't have to deal with this lug/stud extension.
I'm not concerned with the rear running a spacer, as I'm sure it will be fine.