I use this website to get as much info as I can. It works most of the time to provide info on the Spec Build to determine if the truck in question has a Locking Rear Diff or not.
Go to:
www.vindecoderz.com, then enter the VIN #, once you get the decoded information just do a Screen Search (CTRL+F) for "diff" and it will take you to the spot you need to look.
You will see this if Locker = Rear Axle Differential - Rear Axle Locking
You will see this if No locker = Rear Axle Differential - Rear Axle Open Diff
For some reason not all VIN #'s produce the full page of info, but it's been accurate for every known Locked VIN# I've ever checked.