I've been there... drained the tranny fluid, THEN went to refill and...OOPS.

If you haven't already drained your tranny, then first thing is to go get a spare fill/drain plug to replace the one you've mashed.

What you have going for you is that the trans case is aluminum and the plug is steel. They both expand due to temperature, BUT at different rates. Aluminum grows at about 13 parts per million per degree F, steel at about 6. Warming the case about 100 to 150 degrees will reduce the interference between plug and case by about 0.001 to 0.002 inches - which may be enough so that vise grips work.

Since I had a drained tranny, driving around to warm it up was not an option. I used my Milwaukee industrial heat gun to heat the area around the stuck plug. You don't want to go crazy here... and DON'T use a torch! The flame applies heat in a too-concentrated area. Aluminum starts to lose its material properties at temps as low as 350F depending on the alloy, so be careful. You also don't want to get the case so hot that you coke the oil inside. Realistically, I would aim to get it hotter than you can touch (that's about 150) and no hotter than spit boils/sizzles (around 220). I alternately tapped the plug with a hammer and tried the vise grips, and eventually that bastid came out.

Be patient, and remain optimistic. Don't get frustrated and grab the plumber's torch.

When you put it back together, you can use the never-seize, but that's a personal choice. It's not a matter of the dissimilar materials galling, it's a tapered thread that seals by interference. Just don't overtighten it.
--
Thank goodness we don't get all the government we pay for. -- Wiley Post