Change the order of the divs in your xhtml files
Now it’s:
<div id="content">
<div id="main"></div>
<div id="left-menu"></div>
<div id="rightbar"></div>
</div>
change into:
<div id="content">
<div id="left-menu"></div>
<div id="main"></div>
<div id="rightbar"></div>
</div>
Peter
depending on your theme: index.php, single.php, page.php, category.php, search.php, and where these divs could be.
It’s possible that these sidebars are included (or called by a function get_sidebar(), in that case just change the order of those.
Peter