There's nothing strange about requiring JSON. It's a server standard at this point. My guess why you don't have it enabled is due to being on PHP 5.1 (massively out of date PHP build). I've never seen a server that didn't have it enabled. Without it you can't make safely formatted AJAX data calls.
We can't fix this for you, it's a server issue that your host needs to fix. Simply contact your host and ask them to enable JSON for your server. If your host won't do it for you then you need to contact a server expert who can. If you want to enable it your self then please see the below as there's several detailed instructions around the web to do so.
www.google.com/search?btnG=1&pws=0&q=enable+json+php
If the module exists it should just be a matter of adding the below to your PHP.ini file.
extension=json.so
Note CURL is also needed, but appears it's already enabled.