Code in the glove box will be G80 if the truck came from the factory with the Gov-lock. Best to open up the housing and look. Gov-locks in the 10 bolts do tend to have issues, pretty hard to test them. You can try both wheels in the air, rotate one (by hand) as quick as you can. If it's working right, my experience has been that you'll get it to lock.
There have been many posts about the gov-lock on here, and how they work, and how they fail, you might want to research that. When they work, as long as your wheelspeed isn't that high (over 25MPH) when you wheel, then you should be fine. If you need wheelspeed, they are no good for your application.
Besides the fact that even under throttle they aren't locked except under certain circumstances, so if you need traction all the time, a locker is the way to go. Truetrac/posi's are ok, but are still a problem if you end up with much of the vehicle weight only on one tire of that axle.