[eluser]PhilTem[/eluser]
It's simple: You can't.
Though this answer sounds harsh, but without any further fixes it's not possible. Everything that you want to access via an URL/URI needs to be at the same level or below webroot level. The only possibility would be creating an alias (htaccess or apache2-conf) that points to another directory. However, files will then be directly accessible via the alias.
Therefore: For AJAX every file needs to be accessible via an URL/URI