You could use the position() method to get unique ID's like this:
Code:
<table id="careers" border="0" cellspacing="0" cellpadding="5">
<xsl:for-each select="$currentPage/node [string(data [@alias='umbracoNaviHide']) != '1']">
<tr>
<td valign="top">
<br></br>
<a href="javascript:void();" onclick="show_hide('col{position()}')">
<xsl:value-of select="@nodeName"/></a>
</td>
</tr>
<tr >
<td id="col{position()}">
<xsl:value-of select="data [@alias = 'CareerPosition']" disable-output-escaping="yes"/>
</td>
</tr>
</xsl:for-each>
</table>