Changing the rear brakes the other day, and while the wheel is off I am inspecting the rear end and drive shaft.
I look at the end of the drive shaft and see its damp and a tiny drop is sitting on the bottom of it. I wipe the drip, could not get an accurate smell of it because in the process I wiped some rust onto the rag.
So the fluid was unknown. But there has never been a leak where my truck is parked at night.