Likewise I put 2 batteries through my car yesterday and the car was great, even with very old Sorex 36s. I havent had any diff problems but a friend of mine has. Not sure exactly the problem but I believe he had one of the shims 'cone' on him which occasionally locked up the diff. I had a pin in one of my rear CVDs come loose (forgot to check/tighten when I built the car - user error) and start to destroy a rear hub but that was quickly fixed.
Gave a few of my club mates a drive and they all enjoyed it. Heaps of steering, very resposive and easy to drive fast. What all good RC cars should do
Simon