I have had several GM rear differentials that have done this tight turn ****** and jerk, most notable was a TransAm that would bark the tires when turning it jerked so hard. The Transam was serviced and they DID NOT add the limted slip additive that it has to have. It was a limited slip rear end that required it. I am going to make a guess here based upon my past experience. When the rear differential on your vehicle was serviced they used the limited slip additive in yours which should NOT be used in a G80 which I think is what you have? First check your differential type for G80 code and then I would be checking my differential service ticket very closely. If they did in fact use the limited slip additive in a G80 they need to flush the differential and install proper lubricant without the additive. If that is what is going on you do not have an emergency just get a proper service done ASAP. Independent Shops and Dealership service departments can make mistakes and I think it happens pretty regularly.............. That is just my guess here and what I would do first.