Bevel gears?

[Thirdgen89GTA]

so you would put the shim between the bearing and the diff cup?

It all depends on what needs shimming. The reason this is so difficult to explain is that the tolerances are different for every car, even of the same make and model. Wear and tear changes it. Shimming the diff cups moves the diff laterally (left to right). This controls how close the teeth mesh.

Shimming the pinion move the pinion longitudinally (front to back). Most times you will not have to adjust this, but occasionally you might have to. Avoid it for now.

The best I can say is read the article posted by J-dub. It explains it in more detail, and it has pictures of the components.

If you want more detailed information look up info on how to shim a full size cars differential. That will give you more detail than you could ever hope to use in an RC. If not, your best bet is to ask someone in person the next time you are at the track.