I've decided to to try a different tack, giving up on ajax and/or ahah. I'm now trying to send my constructed url using drupal_http_request($url,$header). This results in a 400 error. I know the url is correct because I can clip a copy of the string into the address bar and fetch the resulting json into my browser. My function looks like this:

function get_search_results($url) { $header = array(); $header=array ('Content-Type'=>'text/xml'); $result = drupal_http_request($url,$header); drupal_set_message($result->code); }