What is the correct tool to use to remove the filler plug? It looks like the drive from a socket wrench will fit but it feels a little loose and I don’t want to round the plug off.

Alex

Well, the drive from a socket wrench is the tool used for that. But then again, the things are so stuck in the diff-cap normally that you should take a tool that fits best.
You could try to heat up the whole thing or use a very long piece of pipe for leverage. Then it will go open with a *pop*.
You should always open the filler plug before you open the draining plug, just in case