Get Div's Text in Drupal Forms API
An AJAX callback is not a submission callback; the submission callback is the usual one a form uses, independently from the form using AJAX or not.
An AJAX callback is given the $form and $form_state parameters, allowing it to produce a result, which it returns for rendering. Since it gets $form as parameter, accessing that value simply means parsing $form['id']['mypreview_modal']['#markup'].

function myid_user_page_form_ajax_submit($form, $form_state) {    
  // Parse $form['id']['mypreview_modal']['#markup'].
}


Keep in mind that usually an AJAX callback just returns the part of the form that needs to be rendered. See poll_choice_js() as example.

function poll_choice_js($form, $form_state) {
  return $form ['choice_wrapper']['choice'];
}


The rest is done from the submission handler. (In the case of the Poll module, and for the form using the previous AJAX callback, this is poll_more_choices_submit().)

  // If this is a Ajax POST, add 1, otherwise add 5 more choices to the form.
  if ($form_state ['values']['poll_more']) {
    $n = $_GET ['q'] == 'system/ajax' ? 1 : 5;
    $form_state ['choice_count'] = count($form_state ['values']['choice']) + $n;
  }
  // Renumber the choices. This invalidates the corresponding key/value
  // associations in $form_state['input'], so clear that out. This requires
  // poll_form() to rebuild the choices with the values in
  // $form_state['node']-&gt;choice, which it does.
  $form_state ['node']-&gt;choice = array_values($form_state ['values']['choice']);
  unset($form_state ['input']['choice']);
  $form_state ['rebuild'] = TRUE;