Weird...
I dont have had any problem with my diff, but a normal wear, after many, many pack and 2 races.
I'm sure you built them correctly, I'm sure you set them up correctly, so... I don't know where is the problem, but the first thing I'll change for my next diff will be the grease. I don't know how bad is the stock grease, but I'm pretty sure that any other brand (AE, Shum) grease are better.
If you can't fix the problem, try to flip out the ring, make attention about the thrust and try to rebuilt your diff with a good diff grease and a good thrust grease.
Or, as you say, buy the b fast kit diff, with a set of lub
