Looks like it caught a rock like mine. My limited-slip FG diff has a big rib all the way around in the middle so it probably helped limit the damage.

If the truck moves freely without any rubbing noise from the diff then you probably just need a replacement sleeve. Once you get the old sleeve off the diff internals might be in good shape. The grease might have dirt mixed in if the previous owner ran the diff in that condition in the dirt allowing dirt to get inside. If that's the deal then you might need to clean out the old grease and put in new. If you can avoid it try not to take the bevel gears out because it is a real pain in the neck to get it back together.

You might be able to use the sleeve that comes with the plastic diff on that alloy unit. The two sleeves look very similar.