If you have a dremel tool, you could grind some straight edges on the bolt head to get some pliers on it. You could also try fitting on the next smallest socket in metric or SAE. If you are not willing to try it yourself, take it to a shop for extraction. JNZ or Extreme PSI will carry replacement fill and drain bolts. Do not forget the crush washers for them.

I'd order a new fill plug and washer, and while ur waiting for it, weld a nut or bolt to the current drain plug and back it off like so.

The part that the bolt is most rounded off at is at the top of it, not really any space for a dremel but its something ill try tomorrow, and for the welding, I dont have a welder and plus theres like a sway bar right infront of the refill bolt so not much space there either.

Take the sway bar off to weld something on if all else fails its not that hard to remove the rear diff and a shop could extract and replace the bolt for an hours labor if that.