There is absolutely nothing wrong with the stock Losi or AE diff parts. The fastest team guys - RC, RM, Tebo, Truhe, Robbers, etc, etc. all run the stock stuff guys.
New rings are fine - but if you are really picky - get some 600 grit sand paper and lay it on a smooth flat surface and wet the paper (I use my glass stove top in my house) - put a diff ring on the female out drive and sand the ring in a circular motion until the ring has a uniform finish or if a used ring, until the track where the diff balls ran is no longer visible. Do both rings - assemble - you are done.
One thing worth mentioning is make sure you have the newer style thrust bearing if you are building a Losi diff - the one with the caged balls and the thick washers with the concave races for the balls to run in. They came with the AD2 and the JRXS stock...and I assume now with all the cars.