The best way to know the proper interval would be to do a few UOAs. Perhaps run 30k miles and take a sample; check the vis and wear metals and TAN if so inclined.
Since these axles don't come with a TrueTrac in them, I presume you replaced with OEM diff (a G-80 gov-bomb?) with this Eaton TT? How fresh is the work? I ask only because you may see elevated metals if there were new gears installed with the diff. If the gears were reused, the wear won't be quite as high as they will already be worn in.
Yes - you can use syn fluids, but they will alter the torque-bias you are seeking. Same goes for FMs; you don't want any fluids that have LSD (aka FM friction modifier) in there.