Spray a good penetrating oil in the keyway and work the key a dozen times. Repeat. Also do the door latch mechanism. You probably have old congealed oil and dust gunking things up. A good penetrating oil will dissolve it and flush it out. Be generous with the penetrating oil!
If that doesn't do it, take off the inner door panel and lube everything in there.
If that fixes your problem, follow up with a good lubricant. Penetrating oils don't generally work well as a long term lubricant.
If that doesn't fix it, you might need a new cylinder. Good time to get a new key pattern, so you don't have
a truck with 100,000 copies of your key floating around.
I replaced all three (both doors and the ignition) with
a new set from GMCPaul's, and it was an easy job, though I"d never done a lock before. I ordered mine keyed alike instead of having the standard GM door different from the ignition.
PRO TIP: Get some
new clips if they don't come with what you order.