I have 3 center diffs and 3 front/rear diffs. centers are 5-7-10 and the front/rears are 2-3-7 plus the ones in the car, 7-5-1. Its alot easier to sway out the diff that drain and refill at the track. Swap them out and then rebuild the one you took out when it needs to be.
Tires are a good starting point, diffs can get complicated if you let it. See what others are running diff and tire wise, start from that and build from there.
I'll see if I can find the artical on tire selections and what diff oils can do for the conditions, its liable to help out more.