TRANSFER CASE and DIFFERENTIAL SERVICE
Like most any other fluid in your vehicle, transfer case and differential oils degrade over time and begin to loose some of their lubricating properties. Just like motor oil, they need to be changed at the specified interval.
Some differentials are also equipped with limited slip which requires special additives which if neglected will cause premature wear to friction material.
Transfer Case Service should be performed at least every 30,000 miles on most 4-wheel drive vehicles.
Differential Service intervals vary considerably from vehicle to vehicle but a 30,000 mile service interval is recommended for most vehicles.