BTW, the vinyl is XXXMain Ice internal graphics.
Lars, when you sand the diff, ensure that the surface you're using is very flat. I used our kitchen counters since they're granite. Sand a little, then check. You'll see that the area you've sanded is matte looking. The areas that have not been touched by the sandpaper will still be a bit shiney. Keep going until the entire mating surface has a matte look to it. I'd start with 400 or even something a little rougher. 600 seemed to load up with the plastic too quickly. I'd say once get get everything nice and flat, finish up with 600.
I sanded mine and it did not fix it. I tried sanding twice to ensure it was flat. Simple fix for me was some silicone rtv on both sides of the gasket. No leaks now.