What type of truck do you have?
I've never done it before but looked into it once. Yeah you have to pull the hub do get the brake disc off.
Do you have a FSM?
If you need a diagram for putting things back together.. From my 83 FSM:
As for the gunk that looks like CV grease to me. Not sure if it's supposed to be all over the inside of the hub like that, as I've never broken into one, but my gut tells me no. Someone on where who has should be able to chime in though...