I took a cheap socket wrench i didn't mind breaking and used my floor jack to push up on the wrench.

Doing this used the weight of the car against the wrench to crack open the filler plug. Definitely worked in my case. It didn't take a lot of pumping of the jack to open the bolt, so be careful to not do anything stupid.

If this doesn't work for you (highly doubt it) than you can unbolt the differential cover and remove it entirely. Put it onto a bench and work against it standing up.

But try my suggestion first. That's worked for me every time.

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-Typos courtesy of my mobile phone.