I had an experience I will not soon forget. I walked out one morning to find my (locked) tailgate missing off my 2010 Silverado. After spending $500 for my deductible and 3 weeks waiting to get a replacement, I found something that is so simple, it is pretty amazing. McGard
makes a tailgate lock (item #76029) McGard Tailgate Lock
that goes around the right side hinge. It took me all of 3 minutes to install mine, and seems to be operating fine. It is out of the way, and takes about 3 minutes to uninstall, should I need to remove the tailgate. I got mine on ebay for about $25 after shipping.
I knew they made reliable locking lug nuts, but this is truly as genius as it is simple. With the same technology as the wheel lock, you have a computer-generated key that removes the tailgate lock.
The guy at the body shop said that although mine was locked, it can be defeated it someone breaks the housing around the lock and handle. I don't know if it will keep mine from walking off again, but I feel better about my chances.