Rear Differential Refill/Whine

Hey, what is the tool needed to unscrew the filler plug for the rear axle? At first I thought it was a large allen key but it doesn't seem like it. Also, with the mustang level will the fluid leak out of the filler hole when I pull the plug? I ask all this because my rear clutch packs have started to whine and moan like a who*e so I'm thinking of adding some XL-3's (Too bad XL-3's don't work on mine when she's complaining :Zip2. The only thing is I'm thinking about going the distance and just taking off the housing cover and drain it all because I'm not confident on whether the dealer that swapped my housing used the right grease/friction mod ratio awhile back. Opinions?

Thanks a lot. It does fit. However, do you know if there is an actual square 3/8" key because the ball bearing in the extension made it really hard?