Front and rear differential fluid as well as transmission fluid are recommended to be inspected every 30,000 miles. (samarins.com)
FYI it can also mean your differential is low on differential fluid, though, that might only be the case if it's a very aggressive axle leak.. (mechanics.stackexchange.com)
The oil was changed at 500 and 1,500 miles, and the rear differential fluid was replaced at 2,000 miles. (bringatrailer.com)