I don’t really understand the mechanic’s description of what went wrong. I read a service bulletin from GM (when replacing the fluid in my truck) that the GM 1/2 ton axles equipped with a locker don’t need any additive. I didn’t use any and didn’t have any problems. And gear oil would coat the gears pretty much as soon as the truck was driven, I would think. Hopefully they get you back on the road, though. Let us know what happens.