List only grandparent pages

I’m trying to create a somewhat complicated CMS-like navigation system for my static pages. Here’s a basic overview of the page structure:

Home
About
Products
-Things
-Items
–Different types of items
–Origin of the names of items
–Etc.
-Widgets
-Dongles
Contact

So, there are parent pages, child pages, and grandchild pages.

In the sidebar I was successfully able to display only the parent page and only the children. That was easy:

<?php
	if($post->post_parent) {
		$children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0&depth=1");
		$parent = $post->post_parent;
	} else {
		$children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0&depth=1");
		$parent = $post->ID;
	}

  echo "<li class=\"section-title\"><a href=\"" . get_permalink($parent) . "\">"  . get_the_title($parent) ."</a></li>";

  if ($children) {
  	echo $children;
  } ?>

What I’m trying to do now, though, is have a separate navigation bar when there are grandchildren pages that only shows up on those grandchildren pages. In essence the same thing as what I have above but up a generation.

I tried using get_page_children since wp_list_pages doesn’t seem to allow multiple child_of parameters to be passed (or a grandchild_of variable). I tried using the code below, but it doesn’t work.

<?php

			if ($post->post_parent) {
				$children = get_page_children($post->post_parent, "");
				if ($children) {
					$grandchildren = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");
				}
			} 

			if ($grandchildren) {
				echo $grandchildren;
			}
		?>

What’s the easiest way to move my working parent-child relationship up to child-grandchild pages?

ThankS!