I did end up owing, and I did end up with a refund on my state taxes, but it's not quite enough to cover what I owe federal. Luckily it's only about 300 dollars difference, which I have in savings and can cover without issue. I guess that happened because they took out taxes as if I was married, but I had to file as a single person because we got divorced before Dec 31st (this was news to me).
I'm still kind of glad I underpaid because that money has been earning me interest and saving me from paying interest all year, rather than loaning it to the government interest free. In fact, I think I'll underpay again this year, because I think I can handle paying taxes instead of getting a refund.
So, that's done! And I never have to file with my ex again. Whew.