As promised here what I saw when replacing the front differential oil. Once again I went with M1 Synthetic 75W-90. The front differential was at least as "awful" as the rear. There was a large amount of metallic paste on the drain plug magnet as you can see in the pic below:
It was a little difficult getting a clear picture of the gunk with my phone camera but that large black blob on the towel isn't a smudge but a large glob of metal paste that I half wiped off the drain plug.
I also observed metal particulates in the oil itself, despite the presence of a magnet on the drain plug. I believe this is due to the metal glob on the magnet being larger than it can effectively retain within the swirling stream of oil passing over it.
If I knew previously what I know now I would have changed the oil in both differentials shortly after break-in and then every 15k miles until the amount of break in metals dropped off. Obviously differentials are pretty resilient but I'd still prefer to not run them with metal suspended in the oil.