I just got done changing my fluids a little while ago. I can't remember the socket size for the fill plug, but the drain is a 3/8 drive square deal. The drain is on the very bottom. The steps I took were.
1. Remove fill plug to make sure you can get fluid back in once you drain it.
2. Place a drain pan directly under the diff and remove the drain plug.
3. I let it drain while I went and grabbed my oil and pump.
4. I then wiped off the drain plug and reinstalled, not sure on torque specs.
5. I then pumped the 4qts into the diff, I ended up maybe 1/2" or less below the fill hole.
6. I put the fill plug back in, not sure on torque specs.
I am getting into my own servicing as well. I have also done my front diff, T case and trans. Hope this helps.