How to Change Your Differential Fluid 101
A differential is one of a car's most important moving parts. It's the thing that takes the torque from the car's driveshaft, and distributes it among the driven wheels. Since there's a lot of metal-on-metal contact inside a differential, it needs fluid to lubricate itself and work effectively.
The first step is to get your car on level ground, and raise it on jack stands or a lift so you can access the differential's drain and fill plugs. It's important to keep the car level, even when raised up, to ensure the correct amount of new fluid makes its way into your diff.
The next thing you have to do is locate the fill plug, and take it off. After that, find the drain plug, and unscrew it. Fluid will come out, so make sure you have something to catch it with.
After every last drop makes its way out of the drain hole, replace the drain plug, making sure to use a new washer and a torque wrench so as not to damage the threads. Then, simply refill the differential with whatever fluid your car's manufacturer recommends. Replace the fill plug with a new washer and boom, you're done.