How about rear diff.?
I've done it just with the car on the ground.
First remove the square drain plug with a 3/8" square (bare ratchet or extension). This is a good habit just in case the plug is stuck and you can't refill your diff!
Take off the two big nuts holding the bracket to the subframe (you may need a helper box wrench on the bolts above to keep them from turning). You could probably remove the bracket to cover bolts too, but mine looked very rusted.
Then remove about 10 bolts holding the cover to the pumpkin. Some are hard to get to, but doable. Make sure a pan is under your diff, and get ready for a stinky mess to drain.
Clean all gasket material from the cover and differential. Use plastic scrapers, do not scratch the surfaces.
Optional. get brake cleaner and clean out the metal parts on the diff. Try not to get it on the rubber seals too much. Let it dry for 20 min. Make sure there is no residue.
Get permatex black RTV and make a good zigzag pattern about 1/4" wide that covers all the boltholes on the cover. Follow the directions on the package.
Put the cover back with the wet RTV, finger tighten. Wait an hour or so, toruqe to 25 or 30 ft/lbs (check manual). I like to wait a few hours to let it set before the fluid goes in. Shops would wait a few minutes.
Refill with 80W90 gear fluid only. I like Valvolvine SynBlend (partial synthetic). You can use any brand, but why not something a bit better if you are saving all this money. If you have a limited slip you must add something called FRICTION MODIFIER! from a small bottle. Fill until your finger can feel it thru the hole. I think its about 1.5 qts. Put back the plug.
You just saved $100!
The other way is like what they do in jiffy lube, get a fluid transfer pump at harbor freight and suction it thru the hole, then shoot new stuff back. Of course you will be leaving some old fluid in there, and the gaskets are usually leaking after a decade.