I read a tsb that says differently. #PIP4054d if you want to Google that just to see what it says. Not that I think adding friction modifier is causing your problems.
The G80 locker may be your problem, though. I wonder if they’re totally replacing the locker when they rebuild your differential, or just replacing the ring and pinion, bearings, seals, etc.? I have beaten on trucks with the G80 pretty severely and haven’t had any issues, but they are known to be problematic.
Wider tires=heavier loads on a lot of things. A truck is a system, when you change one thing, it may affect a lot of things.
Even a class 8 rig, you have to spec out everything to match if you change key components. I drove 17 tears for a Canadian company out of Buffalo NY that was notorious for overloading. Our traiers had twice as many ribs as anyone else. We used 1100 x 22 wheels and tires(old sizing equivalent to today’s 24.5.) when everyone else had 10 00 x 20. We had 6" spacers between the wheels , everyone else used 4.5, because we were blowing out too many tires because when we had more that 100,000 lb. payload , the tires were rubbing together " That was just the trailers, the tractors were similarly beefed. We had air pressure controlled interlocks where we could make all 8 drive tires turn as one. If you locked it in on a dry road, you could not turn the truck.
So it started locking up again. The mechanic opened it up, didnt see anything that appeared to be wrong. He put new fluid in. It has been several months and it has not locked up again.
Meanwhile, my brother in law has an 04 silverado that had the same issue. He was going to replace the whole rear axel. I told him to replace the rear diff fluid. He did so and he hasnt had any problems since.
Not sure if its possible that we both ended up with bad rear diff fluid, but replacing it fixed the problem on both of our trucks.