1. Hard to believe OPs tailgate doesn’t have a lock. Pretty much all of them do now.
2. Before making an ass of yourself with the mechanic shop, make sure you’re in the right. Many laws only require reasonable responsibility by the shop. Different situation than if they damaged the vehicle themselves.
3. I looked at a 250mi rad in N Cal on Craigslist and found at least 1/2 dozen tailgates that would fit and a couple that were from the newer model old body style Superdutys.
But no guarantee on color, camera etc. could have a tailgate that holds stuff in the truck for $500 tomorrow. More for matching.