How can I make a div grow with its container?

/* Relevant CSS */

#contentDiv {
      position: relative;
      margin: auto;
      width: 800px;
      min-height: 736px;
      padding-bottom:35px;
      border-style: solid;
      border-width: 1px;
      background-color: #FFFFFF;
}

#leftNavBox-contentPages {
      position: absolute;
      left: 0px;
      top: 64px;
      padding-top: 5px;
      width: 175px;
      height: 702px;
      background-color: #F2F2E3;
      z-index: 100;      
}

<!-- relevant code for the page -->

<div id="contentDiv">
            <!-- #INCLUDE FILE="/includes/small_masthead.asp" -->    
            <div id="leftNavBox-contentPages">
                  <!-- #INCLUDE FILE="/fire_and_rescue/includes/fireMenus-NO_FORM.asp" -->
            </div>
            <div id="textHolder">
                  <h2 align="center" class="pageHead">Frederick County Fire &amp; Rescue Department<br />
                        <span class="pageSubHead">Sub Heading for the Page</span></h2>
            </div>
      </div>

Hi mrcoulson,

Sorry, which div do you want to expand as which other div expands?  This is either very easy or needs scripting depending on the answer.

leftNavBox-contentPages should grow as contentDiv grows.  So, when the stuff inside textHolder pushes the bottom of contentDiv down, leftNavBox-contentPages should stretch to fill the entire left side of contentDiv -- instead of just leaving white space like it does now.

Here's a complicated example with a big, crazy Spry accordion: http://www.co.frederick.va.us/parks/youthsports.aspx.

I'm trying it with jQuery.  I can get the height of contentDiv, but I can't figure out how to get leftNavBox-contentPages to have the same height.

$(document).ready(function () {
      var divHeight = $('#contentDiv').css('height');
      alert(divHeight);
});

Hey, I stopped trying the jQuery idea.  Instead, I'm just using a background image in contentDiv that repeats and appears to be part of the navigation.  The jQuery idea is cool and I enjoyed playing with it, but it never behaved well in IE and I have to move on to another project.

New CSS:

#contentDiv_EXPERIMENT {
      position: relative;
      margin: auto;
      width: 800px;
      min-height: 736px;
      padding-bottom:35px;
      background-image:url(/dry_dock/images/navBackground.gif);
      background-position:left;
      background-repeat:repeat-y;
      border-style: solid;
      border-width: 1px;
      background-color: #FFFFFF;
}

Result: http://www.co.frederick.va.us/parks/youthsports_WORKING.aspx