Check the Periodic Maintenance sticky. IIRC the interval is 30k miles, <1qt GL5 75w90 and it's easier than changing the engine oil.
Just pull the bottom plug to drain and refill through the top one on the back of the diff until it comes up to the top hole. You can get a cheapo gear oil pump at any parts store that will make refilling a bite easier.
Check the Technical Reference sticky in the Powertrain forum for oil options.