Seriously, I would just go with the rubber expansion plug. If it fits correctly, it is impossible for it to fall out. Hell, it may be hard to just remove it. I have spent hours on my back prying them out of engine blocks because they get hard, form a ridge on the inside and do not shrink when they are unbolted. I don't think you will have this problem with the transfer case.

Worst case, glob some sealant on there and be happy with it. To drill and tap, you are going to have to split the case anyway and by the time you buy a bit and tap, you are going to be into it for some $$. Trust me, I spent a long time looking at this problem. Like, a year and a half, while my truck was apart. Now that I think about that, that $150 price was for both plugs. So, it was just down to a minor whimpering over the cost. And if you think about it, that is still only 2 hours of shop time.

If you can find another case, anywhere, do it. The other reason I went the machine shop route was to solve the galvanic corrosion problem. Other that Isuzu having a brain fart and putting a threaded plug into an Al case, they also put a steel plug into an Al case. Outside of the brute force failure mode(i.e, what you did), these puppies just weld themselves into the case, even in the constant oil bath.

As I said, the best is a new case, other than that, don't screw with tapping it. Just plug it and go on your merry way.

Michael