that's when I noticed mine as well, doing my own oil change. here is verdict. They say its residual oil from changing oil. I told him there is nothing leaking from top down it's all confined to axle in the pic. I showed them clear oil which he saw but said they couldn't find anything. Tests they did were= coolant pressure test, engine oil leak test, transmission fluid leak test, 5 mile road test. nothing about front diff. I got a fumoto valve which is not leaking. I let everything drain I'm not messy and it's not used oil color. atleast its documented