The bottom line is that the jquery/ajax code needs to know the url to work as expected. If you are simply trying to hide it from "right click -> view source", try loading it into a variable in another .js file. If you are trying to hide it from users, then thats a lot harder. Anyone with a Firebug plugin for Firefox can trace all AJAX calls including the URLs they were sent to.
Even if you setup some kind of URL re-writing scheme, the user can still request the URL found in your page source and the server-side would not know the difference.
If latter is the case, here is something you can try.
1) Set a cookie with some known value (the return value of time() for example).
2) Pass that same value in your URL.
3) In the PHP code check the value passed in the query string with that set in the cookie and only if they match output the content.