[Kurt S]

IIRC, the axles are the same on a dually/non-dually pickup, just the wheels change. You can just bolt the wheels on and add the flares and you'd have a dually. Ford didn't sell a 4WD dually for a few years (92 or so??) and that's what you did to make one.

And I think Mark is correct that there were two widths of dually axles, pickup and cab&chassis.