You should be able to do this in your style, The easiest way i've found is making a new template, ie/ left_navbar, setting it up as a <td width=150 etc etc></td> put your content in there (your style will change of course).
Then in your forum display forum home and postbit templates put in say $left_navbar in the correct places then edit your global.php and add say
PHP Code:
eval('$templatevar .= "' . fetch_template('templatename') . '";');
Hope this helps.