Outside LOOP...
Code:
<?php
$post = $wp_query->post;
if ( in_category('2') ) {
// do something if they are in category 2
} else {
// show regular menu, or remove this else
}
?>
So what you could do is insert the above code above your regular menu and either include your regular menu in this if statement or do something like:
Code:
<?php
// required for outside of post
$post = $wp_query->post;
// if the current post is in category 2
if ( in_category('2') ) {
// do something if they are in category 2
} else { ?>
this is my regular menu HTML.<br />
<ul>
<li>Menu Item</li>
<li>Menu Item</li>
<li>Menu Item</li>
</ul>
<?php } ?>
Let me know if you need some help.