Rusted rear cables are a very common problem. They are very reasonable to buy in the store new. Little bit of a pain to replace, so I would go ahead and prepare to replace some other things back there also. It would be best to be able to have the truck down for awhile, and then you can tear down one side at a time. It's easiest to change the cables with all the brake stuff out of the way.
P.S. When you are back there, make sure you can push each side of the wheel cylinder inward. I put everything new on my truck except the wheel cylinders, and then my pedal feel was ok but my parking brake would not hold. I finally found out one side of my wheel cylinder was stuck. So two new wheel cylinders and then I was back in business.