Yes, you can use transynd.
Dexron6 or any fullsynth dex-spec'd ATF would be perfect.
If the xfer case calls for ATF, then use any full synth DexronIII there. I don't think that I would use an ATF in a tranfer case with a BB Chevy. I'd consider a 75w80 GL4 MTF/MTL/mesh-type fluid,
With a 14 year old rearend, your posi, if it hasn't been replaced or rebuilt, is probably tired. Fill it with the gear oil you want to use. If there is no chatter, then no additive is needed. If there is, then add an ounce or 2(as needed).