Guard Transmission also makes a great 930 Torque Biasing diff.
I have one for sale that I just removed to make room for their 50/80 LSD.
82 930, 2500lbs 430hp, Black | 85.5 944 Cup / ITS / PCA4 racer, Black
87 Carrera, stock, GP White
00 Audi S4 - 340hp, Imola Yellow
03 Boxster S, GP White