Can the 04 Center Diff Lock mechanism be retrofitted to an 03? Does the 03 have a lockable diff or does it rely completely on the traction control?

 

The 03 doesn't have the correct t/case. If you want to switch your 03 to a CDL vehicle, you would need to locate a older t/case installing it and the linkage into your truck.
I see a couple posts and ads every month for someone selling the old CDL T/cases, buy one install it and then sell yours.
Mike