Ok so I just solved it... Lol!!
Here's how I solved it: did a "Save As" and spun off a second copy of the form. Closed the original, opened the new one (just to be 100% certain). Now I started deleting. I deleted all the form views and then ran the design checker again. Didn't do it; then I deleted almost all of the data sources - that fixed it! (Surprise? Not really). Closed the mangled version without saving.
Then I reopened it and deleted about half of my data sources; the second half eliminated the error. Close without save and reopen.
Did that about 3 or 4 times, deleting and running the design checker. Finally found the suspect field.
As it turns out, I had a dialog box rule for a field, but I had disabled that rule at some point during the design process. Perhaps this is why it didn't take me to the suspect field?
After finding the suspect rule I then reopened the original form and deleted it, solving the problem. So there it is, problem solved.