Hi" we were trying to loosen the differential fill plug but it seemed to be loose as in not fitting socket tightly to get out so wondering what size it is supposed to be, also it was suggested that a screw extractor might get the plug out, AND that this was a common plug sizes readily found at Autozone. Thoughts? Thanks!

I had to drill it out but was able to without damaging the threads in the diff. A replacement was easily sourced at Advance Auto as these rear ends are actually Dana/Spicer derivatives.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)

If you find that the plug head is damage, get a set of bolt removal socket. They have teeth design to bite into the striped head. Just did a job where I had a problem bolt and this tool did the job of removing it very smoothly.
Note: I sure do love six sides box wrenches.

I believe it is either a 12 or 13 mm socket.

Have you sprayed penetrant on it yet, like say PBlaster? If not, I would, and then whack it with hammer to shock the threads. I'd also let it sit for about 15 minutes or so, and then if all else fails, try some heat on it followed by some cold to shock the whole thing.

I've used the following:
http://www.autozone.com/test-scan-and-specialty-tools/bolt-extractor/irwin-5-pcs-bolt-grip-base-set/532381_0_0?cmpid=PS:3:3:1&s_kwcid=AL!5142!3!224896792767!!!g!294682000766!&ef_id=WXsYUgAAAJRCHRKe:20171025145552:s

to get that bolt out.

As far as a replacement from Autozone, get an OEM from a local PNP, or the stealership, not sure Autozone would have it, but then I could be wrong.

Matt