Tailgate lock
Cheapest I've seen is to take a radiator hose clamp and attach it to the side of the tailgate that allows you to remove the tailgate, it takes them longer to steal your tailgate.
Then I tried the PopNLoc on my last truck, got tired of going to the cab and turning the truck off to get the key to unlock the tailgate.
Now on my Dodge I purchased an item, can't remember the name, that covers the opening on the tailgate and is held in place using the factory torx screw. It cost about $20 and a pair of "Jimy Jamerz" for the front doors.
I talked to a local Deputy Sheriff and they caught a 20 year old stealing tailgates in a campground, 5 of them if I remember correctly. Turned out his family owned a body shop and tailgates were a "HOT" item to have.
Hope this helps.