You can leak either gear oil or the inner hub grease which is not repair able.

As for checking the gear oil level, no one makes am eye oil, I am wondering since you work on other trucks, and don't know how to check a diff's fluid level, how often go you service your truck, like a major service?
Pull the fill plug , if fluid comes out, it is full and if when putting a finger inside the hole, if you find the oil is low, you add or better yet change the gear oil with an 85/140.
As for replacing the hub, you will need a torque wrench that goes to at least 350 foot pounds.

 

"You can leak either gear oil or the inner hub grease which is not repair able."

The leaked fluid is not an oil for sure.. It is the thick grease like bearing grease..
Is this not reparable as you stated?? Mike
I ordered an O-ring and Outer seal to see if that will fix the problem.. If not I will need to get a new hub I guess

 

That sure looked like wheel bearing grease to me.
Maybe just clean it all up.
Make sure the wheel bearings are fit.
Put it back together and observe it.

 

The gear oil leaks though the hub carrying grease with it. The o ring will not fix this, it only seals between hub and axle tube.