If you are ready to go to Canada, you could try showing up at the border with your expired PR card and see if they let you in without reporting you for not meeting the residency requirements. If they do, it means you still have your PR even though you don't meet the requirements. In that case, you should stay in Canada for two years straight without leaving and without applying for anything from immigration. At the end of the 2 years, you will meet the residency requirements again for the past 5 years and can apply to renew your PR card without consequences.
However, if they report you on entry for not meeting the requirements, they will still let you enter Canada but you must appeal for your PR. However, education outside Canada is not considered a very good excuse and they will ask if you just wanted to complete your PhD, why did you not try to return as soon as you completed it? You say you gained your PR when you were already in this program and you completed it after 4 out of the 5 years of your PR card so you could have returned for the last year while your PR card was still valid.
If you do not want to try your luck at the border, you can apply for a travel document from the US and list your reasons there. If they refuse which they probably will, you will lose your PR. You could of course apply again if you qualify under some immigration class. However, most immigration classes today need a job offer.