All of those dual rear wheel Rangers which you see have been custom built, Ford didn't offer a dual rear wheel Ranger here. There was a few conversion companies that offered them through Ford dealers, but they were more along the lines of a conversion type of package.
There were a few companies that offered dual rear wheel kits for small trucks. J.C. Whitney still offers the adapters.
Judging by those that I have seen, the adapters are not very heavy duty and seem to have a tendency to self loosen. In other words they seem to be more for show than for any other purpose. I have seen complete kits, but not for a while now, they offered fenders, adapters and wheels. I have also seen front adapters which alloy you to use 6 identical rims too.
http://www.kingstonbid.com/_disc18/0000022d.htm (Found online)
http://tinyurl.com/v3nlq (Comes in 5 lug, bolts to rear axle and allows the use of deep dish dually rims).
I wouldn't mind seeing a Ranger diesel myself, maybe not a 6.0L. but any suitable turbo diesel that makes good power would be great.