I'm needing to get in touch with the web developer on my team as I don't have GTM access. I sincerely appreciate your help to this point. Once I can look into it with my web developer (shouldn't be too long) I will post what we've found out.
I think we've fixed this. I don't know too much about JS, but with the help of the web developer on my team we found a tag that seems to have been the culprit. It was added by a consulting agency to which our search marketing team had given GTM access. It was to track Marketo form submission on their end (for their SEM landing pages). We're going to need to tighten that up a bit.
Thanks a ton for your help Sanford!
Erek for the next time someone posts about this, I wrote up a longer explanation: Why do I see my form fields in the URL, instead of seeing the Thank You page?
Thanks again Sanford!