[Lynne]

There are Replacement Variables, but those are site wide, not just for posts. You can write a simple plugin to do a search/replace for words in posts:
location: postbit_display_complete

PHP Code:

$word = array(
'word1',
'word2'
);

$link = array(
'<a href="/forums/link1.php">Word1</a>',
'<a href="/forums/link2.php">Word2</a>'
);

$this->post['message'] = str_ireplace($word, $link, $this->post['message']);