You can look at the RPO codes in the glove box to determine if the truck was equipped from the factory with a locker. There will be a white sticker in the glovebox with a listing of codes. G80 is the code for the locker. You’re supposed to use 75w90 gear oil with no additional additive.
Anyhoo, from the original description, it sounds like the locker isn’t releasing. I’m not sure why, and don’t have much advice other than taking it back to the shop that did the repairs. I’ve swapped entire axle assemblies a couple of times. I’ve never delved into the differential innards other than changing gear oil.
There’s really no reason that I know of why you’d be having repeat problems with a rear differential other than something inside isn’t being replaced or set up correctly when it’s being rebuilt. Good luck.