How do you remove the square head filler plug?

[Chevyman680063]

I'm trying to remove the filler plug/bolt on the rearend to add some more fluid but the square head on it is giving me a hard time. I tried removing it with several different size sockets and pliers but that's just wanting to round off the plug. So does anyone have any suggestions how to remove it or is there a socket or adapter that will remove a square head? Thanks for the help.