What's the specific problem? Why do you need the results to be ordered? If you really do need to order the results, just attach a sequential ID to each request, then hold a priority queue of results sorted by request ID. Pseudo-code follows:

  next_request_id = 0;
  next_response_id = 0;

  // the following code assumes there's a PriorityQueue class implementing a
  // priority queue with push(object, priority) and pop() operations
  response_queue = new PriorityQueue();

  function send_request(request) {
      request.request_id = next_request_id;
      next_request_id++;
      ajax_send(request);
  }

  function receive_response(response) {
      response_queue.push(response, response.request_id);

      while(!response_queue.empty()
              && response_queue.top().request_id == next_response_id) {
          handle_response(response_queue.pop());
          next_response_id++;
      }
  }

The above code isn't very robust as it doesn't deal with requests that die in transit and never return a response, etc.