I recommend Amsoil Severe gear 75W-90 for differentials w/wo LSDs. Or mobil 1 75W-90
For lockers and LSD's I recommend, Amsoil Slip-lock additive.
Last fluid change you will ever need with the Amsoil.
If you want to change the fluid, simply drain it, filiter it through a coffee filter and re-use. betacuse this is a pure synthetic, water wont mix, The coffee filter will remove any water and harmful debris and make it safe for re-use.
Yhe mobil1 is cheaper but a mineral based product that will need to be changed more often.