85-140 will be full synthetic, so that's good. I've never had an issue with Mobile-1 foaming. I'm sure there's stuff that's better, but I wouldn't not drain it out if that's what was used.
Normally, oil is the diff is picked up by the ring gear, and slung around to fit the pinion. This makes a relatively smooth flow. If it's too full, the carrier (which is what carries the ring gear and spider gears) can pick up oil too, but it's more like trying to mix cake batter with a propeller, so it aerates the oil and makes it increase in volume. If this is what's happenning, it's not a big deal, and will eventually puke enough out it's not over-full anymore. But, you can also just pull the fill plug. If oil comes out, it was overfull.
Has your axle ever been submerged in water? If you shock it with cold water, it can suck in air faster than the vent can manage, and it'll suck water past the axle seals. This will also make it seem like it's over-full, but this is a problem, since most of your axle parts don't like water. That contaminated oil will look milky when you drain it, but you can't really see that when it's puking out the vent.
Anyways, here's a good video from Banks showing what's going on in there:
https://youtu.be/9-yG3D3JBRs
Linear Mode
