whoa guys, lets not panic. I noticed this with the 4Runners as well. For some reason, the rear diff gets dark but I feel this is a normal condition and don't worry about it. In every Toyota 8" rear diff i've seen, the rear diff oil is dark even with few miles on it.
I would say to change it more often if it bothers you but I am not convinced there is anything wrong...
maybe someone can shed a bit of light on the matter but don't panic just yet
