I bought a whole set of metric allen head sockets at Harbor Freight for around $10 and they worked great during my diff oil change. Make sure that you can loosen the fill plug before you remove the drain plug or you will have a problem refilling if it's seized up.

I had to buy a pump-style oil can to refill it as there is no way to get a bottle up there with the tight clearance. It took approximately 1.5 quarts to refill until it started oozing out (I used Valvoline 75w90 synthetic in mine). The original fluid looked quite dirty after only 10k miles, and measuring the fluid showed it was 1/3 quart low from the factory.