Man I must be lucky... In preparation for dropping the exhaust to replace the driveshaft guibos and center support bearing, I figured I'd try to remove those nuts the other day when I was already under my car for something unrelated. I thought that way I'd know what kind of a day I'm in for when I go to do the job...
I took one look at them and thought surely I was f*cked, but I was astonished when they zipped right off with the impact. This is after 200k miles in a climate where the roads are heavily salted about half the year. Looking at the front of the studs, I thought the corrosion I saw was the remains of what were once threads, but after removing the nuts I saw the threading was almost entirely intact. To be fair, the nuts on mine were in better shape than OP's, but still: I needed a 13mm for two of them, a 14mm for the third, and a 15mm for the fourth...
Apparently these are copper nuts, which might explain why they came off at all. I know this doesn't help you, OP, but I thought I'd share my story in case anyone else thinks they're screwed and just starts drilling.
FWIW, I was trying to find that exact same P/N the other day too... I'll let you know if I find anything.
The copper nuts:
