And the Fx4 31spline is one unique width bastard. Can't just get a replacement shaft from the JY off an expo.
But yeah, already answered but I'll go ahead and answer too... Fx4's and anything '09 model year and earlier have drums. Fx4's and most higher models have 10" drums. 2010 is the first model year of Ranger to recieve discs in the rear from the factory. You can go explorer axle as they're ALOT more plentiful than an Fx4 31spline, and you now know what's involed with installing the explorer axle noted above. Plus, it'll get the front and rear trac widths more matched. You can visibly see the difference in track width when a stock axle is back there(at least on a 98+ truck). Plus, you have readily avaliable replacement parts for the expo axle(most notably, the axle shafts). I regret going to the Fx4 axle in my '01 Ranger. Live and learn.
And....if your interested, I've got a 30,000 mile used Torsen off an '05 LII. Could throw that into an Explorer.