Differential - Checking Fluid Level
The large flat bolt on the back of the diff is for checking/refilling diff fluid. Its a 1 1/8 inch (imperial, not metric, but somewhere around 28-29mm).
I didn't have to jack the car, however with the car flat on the ground, its hard to get a bottle in there to refill it.
Recommend either a socket with a breaker, or a ring spanner. Be careful where your hands are, because it'll suddenly turn and theres a lot of very hard metal to hit against.
Once its out, stick your finger in and see if you can feel the fluid, it should be just under the bolt hole. If it isn't, top up the fluid through this hole. The fluid is 'hypoid gear oil' or 'differential fluid' for single spinner diffs, and 'limited slip differential fluid' for LSD diffs.
Try and buy a bottle of it with a pull out filling tube, and a 1 litre is a lot easier to use than a 2 litre. Basically fill until it starts to seep out the back.
To totally change the fluid requires removing all the surrounding bolts (apparently 1/2 inch) and taking the rear plate off, then resealing it. I haven't done this, so if someone who has could put up a quick guide here, it would be helpful.