Everything
@Blue Ox said, I'll X2 that. I'd still go with the Spartan locker if it were me, especially since it appears to a designated plow truck. Don't sweat the cleaning out of metal. YES, you're going to get some metal out of it but once you get those spiders out, you'll have alot more room to spray Brake Clean in there and flush all that **** to the bottom of the diff house. Then just scoop it out with your hand. Blow it out with an air compressor after the brake clean rinse and digging it out of the bottom, them get a strong magnet and push it around in there best you can and any tiny stuff left will get on the magnet and pull it out. For real, you're going to 95% of that crap out of it. The other very fine stuff will just become part of your gear oil and being a used rear axle, I can promise your tolerences are already loose enough to handle some very fine glitter floating around in the oil, and you can also lay a small magnet in the bottom of the diff house to keep all that crap to the bottom if you want too. So don't let the clean out scare you. You know it's got miles on it and it's time is limited before it needs to come apart for all new seals and bearings.
So I'd go with the Spartan now. That will help your traction imensley and in actually should extend the life by spreading the strain to 2 wheels instead of just one and allowing 1 wheel to get away. Then when it does go for good and gives up the ghost, that Spartan will still be good and I'd put it right back into the new axle build. Should get through this winter for sure and maybe even next winter if you put oil back in it. you obviously need a pinion seal as it is. So I'd go with the Spartan now, and top off the gear oil once a month to get through the season. You might even decide this spring or summer to take it out and build it with new seals and bearings and reset up all the lash on the Ring and Pinion if the gears still look good. In short, just get it together NOW to get through this season at least. Then when you're not under any pressure, like say late spring or summer, take your time and put the new bearings in and reset the lash.
