thank you Damian! that worked like a dream. for anyone else who stumbles on this, i used the functions.php solution thusly:
// move popup plugin from footer to ajax wrapper
$popup = SocialPopup::get_instance();
remove_action('wp_footer', array( $popup, 'print_boxes'), 1 );
add_action('popup_in_body', array( $popup, 'print_boxes'), 999 );
then in my template, just before closing the ajax.load div, i added a custom action
<?php do_action( 'popup_in_body' ); ?>
voila!