I run 2.4mm Ceramic balls, the diff is smoooooooth... until you hold one wheel and the spur. Then its sticky. I think the diff rings are actually sliding on the diff halves. Next time I take the diff apart I will try and resolve it. It has been suggested that I glue the rings to the halves, but I am reluctant to do that.
I have yet to find another 05 with ceramics to find out for sure if thats what causes it. Every one else I know running the car is using the kit balls and there diffs work fine (tho not as smooooth +)