KenshiroU,
You are correct that the Checkout Shipping page (step 1) generates it's displayed text from the module constants that you listed, and indeed all the information diplayed is generated by the relevant installed shipping modules.
As can be seen from the constants, the text is built up in sections, depending on which Display Options are enabled in Admin-Shipping for the particular module.
However, for the Checkout Payment page (step 2), the information displayed is generated by the Zen Cart Order Totals core module from the order info, and your selection in the previous step.
The code you are looking for is actually in /includes/modules/order_total/ot_shipping.php, in the 'process' function, and looks like this -
Code:
$this->output[] = array('title' => $order->info['shipping_method'] . ':',
Regards,
Chuck