As you know that US citizen do not require visa to go to Canada, however, in your case a PR visa is stamped on your Passport and you need to have your PR card with you.
The only solution is that you can either take a chance, or the correct process is to go to the Canadian Consulate and get a permission from them, I am not sure what it is but they will give you a letter, it is not unusual, they usually give this out to people like in your situation. Once you take that, you are good to go. I am not sure if its a letter or a permit but they do issue that in case you lost your PR card in another country or you don't have one.
Hope this helped..