Northridge has a great deal on diff covers. 2 Riddler covers, lube locker gaskets in a package deal. I would have dumped the contents of my rear diff all over the bottom of Strike Ravine if I had still had stock diff covers on when I was in Moab.
As far as rock rails, find something you like from a higher-end manufacturer like Off Road Evolution, Poison Spyder, or Hanson.
Or if you're on a tight budget, look for a set of take-offs from when someone upgraded their Rubicon's rock rails. Or ACE fabrication makes some inexpensive ones...but understand that you don't get "something for nothing". Companies get a lower price point by compromising on design or materials (in general). The more of your JEep that the rails protect, the more that you are likely to pay.