First, thank you to all who responded.....This Forum is the BEST!!!
I did pick up a helicoil kit...damn, they are expensive...but I guess cheaper than the alternative.
I checked the M12-1.25 threads against the 1/2-20 thread pattern, with the pitch on the 1/2-20 being 1.27 in metric, and the diameter is 12.7mm....as the saying goes, "good enough for government work" compared to the metric plug that is in there. I had a 1/2-20 tap sitting around, and picked up a 1/2-20 magnetic drain plug from O'Reilly's. The housing hole was pretty well stripped out, the threads came out in almost one piece, but just enough left in there to screw in the old M12 plug, just would not torque down. So away I went tapping in the 1/2-20. The new threads seemed to cut pretty well, put in the new 1/2-20 plug, filled up the differential, and snugged it down...definitely NOT 25 ft-lbs like my manual had, which I found in other posts on here as being incorrect. Drove the T4 around my neighborhood for about 20 minutes checking for any leaking or looseness. I'm going to put some paint marks on the drain plug so that I can visually check it for loosening up. A few more short trips around the neighborhood are on the schedule before I take the T4 on any excursions. If it doesn't hold up...the helicoil repair will be next.
BTW, O'Reillys or Autozone also carry 1/2-20 oversize drain plugs, which appear to have a self tapping feature, but if the standard one doesn't hold up, I don't plan on going that route.
Thanks Gearhead for that suggestion on the paint mark.