At this point I'm just trying to pass it a zip code and retrieve that zip code both in JSON format. Help! I've been working on this for six hours!
By the way this doesn't use a form but fires on event handling. It fires when there are five characters typed into a JQuery modal window with form objects embedded through a <div></div>.
So you've obviously got the jQuery library referenced properly and you are actually calling the function.
Try clearing the cache in your browser, and if you're using a templating engine, delete any tmp files. Try again.
If still no joy, have a look at the php file reference - is it valid? Does it need a prefix path like "/includes/..." ?