New true-trac differential whine

I do my own differential work and always file away photos of the final gear imprint. If I was having a shop do a ring and pinion set up for me I would have them take photos of the final gear imprint and provide them to me. I am surprised they did not do that anyway, about everyone has a cell phone these days. That would save a lot of second guessing if a problem did arise.
To your current issue, thinking that you have an 8.75, pulling the differential and rechecking the gear mesh is not a big deal. Try working with the shop.
Maybe you pay for the lube and gaskets and they provide the labor to pull the diff and re check the gear mesh.
And this time get photos.