The cen nemesis, genesis, and matrix all use the same diffs. Here is a guide to installing: http://www.rcbasher.com/savcendiffs.html

You cannot replace just the fornt or rear, you have to to both. THis is becasue the cen diffs have a differnt ratio from the stock ones, so your wheels will be going at differnt speeds if you don't replace the front and back at the same time.

Herrsavage:
You realize you can use the cups from ANY vehicle with a 6mm shaft (tranny output) or 8mm shaft (diff input), so quality of the cups is a moot point. You can buy the cups from another vehicle, as they all use the same standard dogbones.