2000 Camaro V6 auto bearing whine

Hey again, I just bought a used 2000 Camaro with an automatic RWD transmission. We flew down to Houston to pick it up, and before we bought it they said they had replaced a pinion seal cause an outside mechanic said it was leaking. Its about 250 miles back to Dallas where I live, but about 120 miles into the trip the car began a high pitched whine. My dad shrugged it off as road/tire noise because the tires were mismatched and the road wasn't exactly smooth. The two lanes were actually made of different types of material, so I switched lanes to see if the noise would stop or change, it didn't. So about ten miles later I pulled it over to a gas station and the noise became lower pitched as we slowed down and stopped when the car stopped. We asked a guy where the nearest transmission shop was and luckily it was about a mile down the road. So we took the the car to the shop and they diagnosed that the mechanic had not fixed the pinion seal correctly and probably didnt replace something I think he called a crutch sleeve. They said they would need to replace the whole differential and if I had driven it 40 more miles it would have broken down. I did some research and I think the noise we heard was called a bearing whine but im not sure exactly what that means is wrong or what should be taken to fix it. Luckily we had a 1000 mile warranty on the car and the dealership is taking full responsibility and costs of the repairs. I would like to know if the transmission shop we took it to is correct in that the whole thing should be replaced and if the shop does it correctly will it be permanently fixed, what the noise was if it was not a bearing whine, and what the noise means is wrong with the car. Any response would be greatly appreciated.

Thanks.