Err wouldn't be cheaper to sand the rings down your self? I use 600 wet and dry on a flat surface and a bit of elbow grease and sand the ring by truning them in a large arc, inspect to see where there are unsanded areas those are your low spots. sand until whole ring has and even grained finish. Do both sides the ring are pressed from sheet steel they are not flat. I would then sand them down with 1200 grit wet and dry until you get a mirror like finish. do not polish! the texture surface allow the balls to grip the plates. when you rebuild the diff sand as per the previous procedure the grooves out and rebuild. it take less time to sand the groove out than prep new rings! I do this for all my diffs and use ceramic balls, end up with very smooth diffs that are low maitenace as well, cause they wear is even across the ring it less likely to go notchy, and you don't need to set the diff as tight to eliminate slip. see this
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