Sounds like my balls, er, the 501 diff balls are not perfectly round and that may cause the gritty sound and feel? I'll have to get some of those Kanzen ceramic balls you are talking about. I wonder if they still offer free shipping?
http://www.oople.com/forums/showthread.php?p=28400
EDIT: Looks like the Tamiya diff shield is this? ...
http://www.tamiyausa.com/product/ite...oduct-id=53970