Its not incredibly expensive to do unless you buy Royal Purple for it, so I'd do it ASAP.
I usually do mine every 50k miles or anytime I submerge the rear in water for more than a couple seconds (only happened once so far).
If its milky or white, change it ASAP that's usually a sign of water. Dark is bad but its not as urgent as water issues.
1979 Ford F-250 351M Custom S-Cab Long Bed
2012 Ford F-150 5.0 STX Supercab "Vivian"
1992 Ford F-150 300 XLT Extended Cab "Ethel"