Permatex makes a gear oil specific sealer. Want to say the tube is grey with white lettering? It does work good and you can get it apart without using a hand grenade.

I stopped buying the generic blue and black RTV and have been seeking out products that are more tailored to whatever fluid it's making contact with.

I once put a Ford 8" rear together without a differential gasket and it didn't leak. If the two mating surfaces are flat it's totally possible.