Wow What an anemic tech forum this is.
For anybody here that actually has the nerve to do your own maintenance, The answer is a simple one.
On the bottom of the diff, close to the drain plug is another smaller plug that I'll call the "overflow plug". This plug leads to a channel that is open up to the proper level of oil in the diff. when you refill, put the drain plug back in, but leave out the overflow plug. Next, Pour oil into the fill hole on the right side of the diff (pictured above) slowly until some oil runs out the overflow hole. Put your filler plug and your overflow plug back in place, and you're done.
Or, you can let the dealership do it for you and loot your wallet. Suit yourself.