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After 1977, there was no 4x4 F100(I know my signature begs to differ, but, I made it that way...)
Between a 2wd F150 and F100, the main difference is springs, also, 80-82 power brake F100's have small brakes and the car/Ranger 5x4.5" bolt pattern.
When Ford dropped the F100 nameplate in 84, they simply quit putting F100 badges on light GVWR F150's, you still could get a 4700 # GVWR 2wd F150.
Interesting 82F100SWB, the truck i pulled my rear end from for my 82 was a F100 (1980) with power brakes, and it has the standard truck pattern lugs.
It even matched the front wheels, all the suspension was correct for a standard duty F150.
Not to try and muddy the waters, but i belive some of these do exist out there, and i verified this was indeed an F100 thru the vin plate on the door striker.
Most likley it had power brakes added at some point, all it takes is the addition of the booster and a change in master cylinder.
It's entirely possible that only some of the 80-82 power brake trucks got the small bolt pattern also...
It is also possible that the rear you salvaged from was not the original to the truck. When the Ranchero died in 1979, the only light car like truck was the F-100 which came in all trim levels. There were no 4X4 E or F 100 series vehicles produced, but that is not to say there are none on the road or in salvage as I write this. It is not all that difficult for the determined man to convert his clean F-100 to a 4X4 from a salvaged F-150 or F-250 4X4.