You could write a small snippet that could check the id of the current doc to see if the nav matches and return a class of active to the li or a elements.
I used to do this for all my sites before I started using MODx. Basically I would create an array and a loop that would check match on the document (in that case by a site_section variable) and return the class in the output and it worked very easily. Here is an example:
<div id="menu">
<ul id="navigation">
<?php
//This array defines the links and the site_section names for the links list created by the foreach
$links = array(''=>'home','buy/'=>'buyers', 'sell/'=>'sellers','listings/'=>'listings','team/'=>'Our Team','blog/'=>'Blog','contact/'=>'contact');
// This foreach statement creates a menu with as many links as $values in the array and converts text to first letter uppercase
foreach ($links as $key => $value){
echo '<li';
if ($site_section == $value)
{ echo ' class="active"'; }
else { echo '';} echo '><a';
if ($site_section == $value){ echo ' id="current"'; }
else { echo '';} echo ' href="'.SITE_TOP_PATH.$key.'">'.ucfirst($value).'</a></li>'."\n";}?>
</ul>
</div>
You would have to fix the array so that the value was the id and that the match was on the document id but it would work just the same.
However, you may actually be able to do this even easier using PHx’s advanced syntax something like:
<a href="[~12~]" [+phx:if=`[*id*]`:is=`12`:then=` {{activeClass}} `:else=``+]>$19.95</a>
<a href="[~16~]" [+phx:if=`[*id*]`:is=`16`:then=` {{activeClass}} `:else=``+]>$19.95</a>
<a href="[~32~]" [+phx:if=`[*id*]`:is=`32`:then=` {{activeClass}} `:else=``+]>$19.95</a>
<a href="[~52~]" [+phx:if=`[*id*]`:is=`52`:then=` {{activeClass}} `:else=``+]>$19.95</a>
<a href="[~11~]" [+phx:if=`[*id*]`:is=`11`:then=` {{activeClass}} `:else=``+]>$19.95</a>
You would just place the following into a new chunk "{{activeClass}}:
class="active"
In addition, someone who knows more about snippets MODx would likely be able to write this simpler by merely checking to see if the current document was equal to the url id.
Hope this helps you move in the right direction.
Cheers,
Jay