signupforms.js

jQuery.ajax({
                  type : "post",
                  dataType : "json",
                  url : wpajaxurl.ajaxurl,
                  data : {action: "signupform", form_id : 37 },
                  success: function(response) {
                    if(response.success) {
                      jQuery('#formcontainer').append(response.form);
                    }
                    else {
                      alert("no matching form found");
                    }
                  }
                });

signupforms.php

function stf_grab_signupform() {
  $catID = $_POST['form_id'];
  $formID = 37;

  /*switch($catID) {
    case 'a':
      $formID = 37;
      break;
    case 'b':
      $formID = 38;
      break;
    case 'c':
      $formID = 39;
      break;

  }*/

  $formToShow = gravity_form( $formID, false, false, false, '', true, 1, false );

  echo json_encode(array('success' => true, 'form' => $formToShow));
  $result = null;
  die();
}

add_action( 'wp_ajax_signupform', 'stf_grab_signupform' );
add_action( 'wp_ajax_nopriv_signupform', 'stf_grab_signupform' );