Like Scott, I went fully synthetic with Amsoil in everything (front and rear diff, transfer case and transmission) many years ago and haven't looked back. Had the front and rear diff serviced a couple of years after the switch (just as a precaution) and the mechanic said that the old fluid looked brand new, but I had them replace it anyway.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless