2012 Ford Excursion V10 6.7

Hey all
My good buddy and his fam have been living in an airstream for past 4 years mainly out west, but has driven from ME to Cali multiple times.
They needed a new factory engine 2 months ago, some metal on metal, coolant leaking, the usual.
I just saw them yesterday and caught up.
Looked at the rig before I left and his rear differential had a decent amount of fluid on the outside, from halfway point down. Black. I didn't touch it or do more than the quick glance.
Assuming this isn't normal? 216k body and components
Do I nicely mention this to him?

 

I would mention it.

 

It never hurts to inform your buddy. If he is a good friend he'll appreciate the tip, if for nothing eles to simply keep an eye on it.