This is a very common problem with these trucks. I have seen this may times and it seams to be more common when it gets hot out side. Here are some directions on how to replace the actuator. It takes me about 10 minutes but I would guess it may take you around 1 hour. Its not too hard of a job.
First you must remove the door panel.
Rear Side Door Trim Panel Replacement (SLT)
Rear Side Door Locking Rod Knob Assembly
Rear Side Door Trim Panel Cover
Rear Side Door Trim Panel Cover Retainer (Qty: 4)
Rear Side Door Inside Handle Bezel Cover
Rear Side Door Trim Panel Bolt (Qty: 3)
Notice: Refer to Fastener Notice in the Preface section.
Tighten9 N·m (18 lb in)
Rear Side Door Trim Panel Assembly
Rear Side Door Cable Retainer
Rear Side Door Trim Panel Retainer (Qty: 10)
After you get the door panel off, remove the water deflector. You do not have to take it off completely just pull it out of your way.
Rear Side Door Water Deflector Replacement
Now you can remove the door lock actuator.
Rear Side Door Lock Replacement (Crew Cab)
Inside Door Handle Cable Retainer Clip
Rear Side Door Lock Screw (Qty: 3)
Tighten10 N·m (89 lb in)
Rear Side Door Lock
You have no other warrantys expect the 5yr/100k mile powertrain warranty. Your truck has only had one warranty repair, whick was a recall. You do have one open recall on the truck that should be done, its for the heated washers. You will remove this option due to a fire risk. They will also give you some money back because they have to remove this. Since you have low milage, it is likely GM will help you if you call the phone number I gave you. You will need to give them the VIN and mileage as well.
This is the warranty history.
Job Card Date Job Card Number Transaction Type Transaction Adjustment Labour Operation Odometer Reading 03/23/2009 196739 ZFAT----Field Action Recall V1819 - 08142 ECM EEPROM - Reprogram ECM 7,396 MI 02/21/2008 A04935 ZPDI----Pre-Delivery Inspection Z7000 - Pre-Delivery Inspection - Base Time 0 M