Hmmm... I think I figured this out, but in hindsight, the trouble logging into the admin screen should have been my first clue that something bigger than the shipping module was the problem. When I went to log into my Wordpress admin and the Dashboard failed to load, I finally figured out the problem must be with out host; Wordpress gathers RSS feeds, so best I can figure is that our host's firewall is being overly restrictive and we can't communicate with other sites. A couple of emails to support and the Canada Post shipping module now works!