One way or another, you’ve already ended up with what’s probably the easiest HTML markup to use for the job:

<select size="1" id="Rank" title="" name="Rank">
    <option value="">-Select Your Rank-</option>
    <option value="Airman">Airman</option>
    <option value="Airman First Class">Airman First Class</option>
    <option value="Senior Airman">Senior Airman</option>
    <option value="Staff Sergeant">Staff Sergeant</option>
    <option value="Senior Master Sergeant">Senior Master Sergeant</option>
</select>

and then one <element> container for each possibility of <option>.

<div>
    // For Airman
</div>
<div>
    // For Senior Airman
</div>

… etc etc…

I’d use this same layout for everything that depends on which <option> is selected; the unique-line-of-text you want, the other-select-box, etc etc. I’d wrap each one in a container element, so you can easily target all the elements as one.

<div class="container">
    <div>
        Line of text for Airman
    </div>
    <div>
        Line of text for Senior Airman
    </div>
</div>

<div class="container">
    <div>
        <select>
            <option>Airman Stuff</option>
        </select>
    </div>
    <div>
        <select>
            <option>Senior Airman Stuff</option>
        </select>
    </div>
</div>

Now whack a identifier to each of the <div>‘s we’ve got, so when the “Airman” is selected, we know which <div>‘s are the Airmans (so we know to show those ones!)

<div class="container">
    <div class="airman">
        Line of text for Airman
    </div>
    <div class="senior-airman">
        Line of text for Senior Airman
    </div>
</div>

<div class="container">
    <div class="airman">
        <select>
            <option>Airman Stuff</option>
        </select>
    </div>
    <div class="senior-airman">
        <select>
            <option>Senior Airman Stuff</option>
        </select>
    </div>
</div>

And add the same identifier to the <options>‘s of the <select id="rank">:

<select size="1" id="Rank" title="" name="Rank">
    <option value="">-Select Your Rank-</option>
    <option value="airman">Airman</option>
    <option value="senior-airman">Senior Airman</option>
</select>

Now we’ve got this markup, applying the JavaScript for the hiding/showing is so easy!

$(document).ready(function () {
    $('#Rank').bind('change', function () {
        var elements = $('div.container').children().hide(); // hide all the elements
        var value = $(this).val();

        if (value.length) { // if somethings' selected
            elements.filter('.' + value).show(); // show the ones we want
        }
    }).trigger('change'); // Setup the initial states
});

Done; checkout an example: http://jsfiddle.net/3UWk2/1/

An update for your comment

The reason your attempt to change the code like you did didn’t work is because we’ve currently got no event handler bound to the 2nd level <select> boxes; only to <select id="rank">

You need to basically repeat everything we’ve just done for the first level nav, for the second level nav. Instead of using an #id selector for the <select>, use a .class; because we’ve got more than one <select> element to target, and #id‘s have to be unique:

$('.second-level-select').bind('change', function () {
    var elements = $('div.second-level-container').children().hide(); // hide all the elements
    var value = $(this).val();

    if (value.length) { // if somethings' selected
        elements.filter('.' + value).show(); // show the ones we want
    }
}).trigger('change'); // Setup the initial states

We’ve also had to change the name of the div.container, to stop the <select> boxes hiding each others elements.

Check out an updated example here: http://jsfiddle.net/3UWk2/3/