This is what I think happens...

Did you use any lube on the diff balls the first time you install the diff? I recommend using the Mugen Super Grease. I use it on my Centax thrust bearings and they are excellent. I only rub a thin layer on the diff balls before assembly.

But the trick is you need to do it the FIRST TIME. If you didn't, no matter how much lube or how many times you service it, the diff will remain gritty. I guess when you did not lube it the first time, the diff balls and the plates would already have "flat spots" which will still remain even after you lube it. That's the one that makes the diff gritty. I guess it's ok to have a gritty diff (need somebody else to confirm this) but if you want to make them like new again, replace the balls. Lube it with sparingly with the Mugen Super Grease and be done with it. I believe Serpent is coming out with 4 mm ceramic coated diff balls (1378) for the diff.

I still have mine very smooth as from day one and I have not needed to open it up to service it yet.