Surface rust on 8.25 bearing caps?

Over this past weekend I cracked open the differential housing to the 8.25 on my '91 XJ to change the fluid. Once I got all of the old fluid cleaned out using some brake cleaner, I noticed some light surface rust on the bearing caps. I didn't notice any rust on either the ring or pinion gears or what I could see of the bearing. Haven't noticed any strage noise from the rear other that road noise from an old set of 235 BFG so I'm pretty sure everything is OK in there.

I was just curious as to what might cause that little bit or rust? Has anyone else seen that? The 8.25 on my '00 was clean as a whistle when I opened it up at 55K. I would think that even if some water did get in there, the gear lube would prevent it from causing rust on anything.

I'm not sure what this XJ was put through prior to me getting it about 2 months ago but I think it was used as a hunting vehicle. So, I guess it's highly possible some mosture got inside the diff. housing.

We'll see what the D30 looks like when I open it up soon....

Thanks.

That is not uncommon if the vehicle sat for quite a while. The more important thing is to check the wear pattern and make sure the wear surfaces are not pitted.