You don't need to do that with query strings, the url is good enough.
You will need to study the docs about site configuration to direct users to either a Illinois or Michigan virtual site (siteaccess in ez publish) based on the url.
So if from the origin site you know to direct users to Illinois, let them point to
http://yoursite.com/il/url-to-the-real-content
for Michigan you can use
http://yoursite.com/mi/url-to-the-real-content
Take a look at the URI section in http://ez.no/developer/ez_publish_3/documentation/installation_and_configuration/configuration/site_access
But perhaps it can be done even simpler by using sections/nodetrees (if the content for both is much different)