Loading a post’s content, into a div outside the loop, when it’s title is selected in a list

If the page & content is short enough & you don’t mind some extra initial loading time, you can load all the content tabs on the page at the first time & then just use javascript to show/hide different tabs

// the loop to list the titles here
$wp_query->rewind_posts(); // reset the loop
while ($my_query -> have_posts()) : 
    $my_query->the_post();
    ?>
    <div id="<?php the_ID();?>" class="the-content">
        <?php the_content();?>
    </div>
    <?php
endwhile;

If you strictly want ajax, see this http://codex.wordpress.org/AJAX_in_Plugins

You’ll need to pass the id of the post as a parameter in ajax request & then use this parameter in your code to determine which post’s content is to be returned