my 2005 Scheduled Maintenance guide has changing Diffs and Transfer case fluids under "Special Operating Conditions" (i.e. maintenance while towing) starting at 30K and every 15K miles thereafter.
The maintenance schedule says to "inspect" diff/transfer fluids every 15K miles.
Do a search on this forum for Differential Fluids, etc and you'll find lots of helpful threads. I did this myself a few weeks ago. The rear diff is much easier that the front and transfer case, but they're all relatively easy. Many of the folks on the forum have used Amsoil synthetics (I used their Severe Gear 75-90) which should be good for 100K miles....