<nav class="postNav">
    <ul>
        <li class="active"><a href="#pageHeader">link to header</a></li>
        <li><a href="#one">link to other posts</a></li>
        <li><a href="#two">link to other posts</a></li>
        <li><a href="#three">link to other posts</a></li>
        <li><a href="#four">link to other posts</a></li>
        <li><a href="#five">link to other posts</a></li>
    </ul>
</nav>

var navs   = $('.postNav'),
    first  = navs.first(),
    active = first.find('.active a').attr('href'),
    anchrs = navs.not(first);

anchrs.find('li').removeClass('active');
anchrs.find('a[href=' + active + ']').parent().addClass('active');

var nav   = $('.postNav'),
    first = nav.filter(':eq(0)'),
    notF  = nav.filter(':gt(0)'),
    index = first.find('.active').index();

    notF.each(function(){
        $(this).find('li').removeClass('active').eq(index).addClass('active');
    });

var index = $(".postNav .active").index();
$(".postNav ul").each(function() {
    $(this).children("li").removeClass("highlight").eq(index)
        .not(".active").addClass("highlight");
});

<nav class="postNav">
  <ul>
    <li class="header active"><a href="#pageHeader">link to header</a></li>
    <li class="one"><a href="#one">link to other posts</a></li>
    <li class="two"><a href="#two">link to other posts</a></li>
    <li class="three"><a href="#three">link to other posts</a></li>
    <li class="four"><a href="#four">link to other posts</a></li>
    <li class="five"><a href="#five">link to other posts</a></li>
  </ul>
</nav>

$(function() {
    $('li').on('click', function() {
        $('.active').removeClass('active');
        $('.'+$(this).attr('class')).addClass('active');
    });
});