Having the code is all well and good, but it doesn't really tell you what you need to know. The following is a list of Ford's F150 axle codes and what they actually mean:
If your F150 doesn't have a sticker for whatever reason, there's also a tag on the rear pumpkin that should denote the size. If that's also missing or if you're not sure it's correct, the best way to do it manually is to mark a spot on the driveshaft and then turn one of wheels. All you need to do is count the number of rotations for the driveshaft to get a rough estimate of your F150's gear ratio.
If you have a 4WD F150 and it comes time to swap out gears, you absolutely must get matching gears for your F150's front IFS differential. You can check out our 2011-2016 F150 rear gears here and our 2004-2010 F150 rear gears here.