Decide what you'll use. I went with Redline 75w90 High Performance Gear Oil Find more about what I used here
So, get your stuff
Then lift up the car (in my case that involved cleaning up the bay so my tech would let me use his lift
Locate the rear differential, drain plug, and fill up plug.
You'll need a 14 mm Hex Socket
without that tool, you cannot do this.... sorry....
Go ahead and remove the fill up (top) plug, then remove and the drain plug and flush dirty fluid out.
Put drain plug back, fill up the rear differential from the top using a pump.
Once notice that it overfills, you’re done. Cap it up and call it a day