I got the replacement shifter arm nut that you sent me today.
I just went to swap it out and went from this:
It is sort of hard to see in my photo, but the inner lip on the old one was a bit worn and bent out and the flat washer inside could angle and pop through the hole as it did on me the other day while driving. And looks like mine didn't have the right kind of inner washer either, so thanks for sending that part as well! Looks like my old one has some tick marks on the inner lip and the replacement doesn't. Not sure why they would be different, but seems to not be needed anyway as I just took the old one off by hand and tightened the new one on pretty snuggly by hand as well.
Anyway, this will definitely solve my problem. Thank you very much Vern! I owe you one.