You may just have to find a Mac user and pay them to debug the issue. To clarify for others looking in, it's not that JS won't run at all, it's that the callback isn't being called properly - i.e. you click the "get shipping quotes" button and it submits the form instead of running the AJAX to get the quotes.

There have been other posted conflicts between other JS modules, so you might try disabling JS Tools altogether to see if the checkout page works then. Other than that, I can only recommend you make sure you're on or can update to the latest version of Ubercart.