I recall reading on this forum about someone using a Redline product for rear differentials. You might want to look at this:
SHOCKPROOFTM GEAR OIL - recommended for heavily -loaded racing differentials and transmissions which see shock-loading. ShockProof contains a unique solid dispersion which cushions gear teeth to help prevent tooth breakage and allows the use of lower viscosities. Available as SuperLight, LightWeight, and Heavy grades.