Chevrolet Repair Questions? Ask a Chevy Mechanic for Answers ASAP
The most likely cause of this is that someone changed out the ignition tumbler but didn't re-key the door locks to match the new ignition tumbler. That means the key that you use to start it doesn't match the door. We see this a lot on auction cars. If the truck was ever stolen or repossessed chances are the original ignition and door key was lost. To make the truck driveable the ignition tumbler was changed out.
You can attempt to re-key the door locks to match the ignition but it can be a little tricky if you don't have some basic locksmithing skills.
Do you want to attempt it?
Here is a good video showing how to re-key door lock cylinders.