Yep, you had some moisture in the case. Looks like no teeth missing on the ring gear. Check both gears if you have all your teeth it's not that bad. I would bring it to a local HD truck rebuilder and have them put all new bearings in it, because if they are pitted from rust or the rust residue starts grinding away in them they won't last long and could wipe out the ring and pinion gears (that's bad). Also make sure you pull both wheel hubs and put new bearings in them too as they probably are rusty as well.
Steve