Lately decided to get started on the 30K mile service a little early ( at 20K) and when I saw the syrupy grayish fluid coming out of the rear diff I am glad that I didn't wait another 10k miles! Also changed the trans fluid with valvoline dino 75w90 gl5 at the same time. That fluid still had a nice golden color coming out. I know color is not necessarily indicative of an oils effectiveness but is that common for the rear diff fluid to be grayish?
Initial impressions after the change of just those fluids is that the whole drivetrain seems a lot smoother.