How often should i change transfer case and differential fluids?

I own a 2002 Toyota Tacoma and was wondering how often should i change the transfer case and differential fluids?

Right now its got 120,000 miles on it and they've never been changed.

Most vehicle manufacturers recommend changing the oil once a year or every 7,500 miles in passenger car and light truck gasoline engines. For diesel engines and turbocharged gasoline engines, the usual recommendation is every 3,000 miles or six months.