The 502 diffs are normal, predicatable and just fine if you ditch the stock balls and get the upgrade. Personally, I really like the Tamiya AW grease and find it still very smooth when I rebuild (not that often since I went ceramic). Ceramic is a bit more finnicky than tungsten as it will slip easier. If you look at the crap ball that tamiya supplied with this kit, they probably have very minute flat spots all over them. The good news is the sealing surfaces on the 502X gearbox covers will keep all the grit out of your diffs if you take the time to seal the slots in the outdrives, which extend into the gearbox. I use Durango outdrive boots and they work fine on the tamiya outdrive.
A good set of balls is the key to success
you can see the out-drive boots in this pic.
