Have you checked your web server logs to see exactly what the
query string is? If the parameters are received by the web server
then CF should have them in the URL scope.
Have you used Firebug to monitor the Ajax request? This will
also show exactly what you are submitting. My guess is that your
javascript is not building the request properly.
What Ian says is very true. Cold Fusion does not know what
submits the request to it. It doesn't care. An Ajax request looks
exactly the same to CF as a normal request submitted via the
browser.
Once you get the parameter issue resolved, then the
server-side code is totally up to you. It's just plain 'ol CFML and
you can use a cfc or cfm, it doesn't matter. It's up to you what is
returned to the browser. XML, JSON, plain text, or html.
I don't use ajaxCFC, but it is popular. I'm not sure why you
can't use it, as it is nothing more then CF code. If you can't use
that then I'm guessing that you can't use any third party custom
tags, udfs, cfc's etc. Bummer!