It is factory as the only 4-speed automatic was the AOD. And all of the transfer cases were 2-speed.
The AOD wasn't all that robust and was never offered behind the 351W, apparently because it couldn't handle the power in stock form. But there are people who build it today to handle plenty of power, so if you want to go that way it is possible.
Gary, AKA "Gary fellow":
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Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI