Use the AutoTrac II fluid in the transfer case only,it is a lot better than the ATF.For the rear diff use convential gear lube which is recomended.Problem with synthetic gear lube is that is too hard on ring and pinions tearing them up and my father's friend Rich can prove this.He has replaced at least 3 sets of ring and pinions due to synthetic gear lube used.Use Dextron in the transmission.