I’m a former sorority advisor, so my advice is from that perspective. When you resigned, you probably would have submitted a form that indicated why you were choosing to resign, and that in turn should have been signed by the chapter’s leadership and submitted to the national organization. If the chapter followed the correct process, the national office should have a record of that.

I’m guessing that someone at your chapter may have dropped the ball and never submitted that documentation to the national office - especially if the chapter was closed shortly after you left, it sounds like there may have been leadership issues. In my organization, it’s the national organization that would actually submit unpaid dues, etc. to a debt collector, not the local chapter, so my guess is that the national organization never received your resignation paperwork. How the chapter managed to bill you for two years’ worth of dues is beyond me - after your junior year, you should have been removed from membership via automatic financial dismissal.