The lock cylinder on Tahoes isn't so you can lock the tailgate, it's so you can release the hatch glass without having to push the button on the dash, or if your battery is dead.
So usually, you pull that top handle at the bottom of the glass, and it's locked. Put the key in and turn that cylinder, and the hatch glass pops open.
Sorry, man. Maybe you can search google or forums and see if someone figured out a way to make a locking tailgate.