I've never changed a 9 inch's oil. But i wouldnt think it would be that dificult to do. However, if it is, removing the brakes, and sliding the axles out is one of the easier things you can do to a rear. You'll need a 15 mm socket, or wrench for the calipers, and caliper mount bolts. If im not mistaken, the axles bolt onto the ends of the axles with a square, 4 bolt flange, so youll need a wrench for that. Once those are out of the way, the axle should slip right out. Just give it a gentle tug, and pull it out only about 6 inches. Just take care not to crush the axle seal behind the 4 bolt flange. Just unbolt your center section enough to allow the fluid to drain. Button it all back up, and refill.
Grab yourself a very basic set of tools from the parts house. Get the cheap stuff.
And i'm sure theres a writeup, or a video on howto for a 9 inch. Watch that and give
It a shot. Youll be satisfied more when you do it yourself.