Jeep - Commander :: Permanent Whistling Type Sound After Rear End Work?

I have a tan 4-door 2006 Jeep Commander with a little over 100,000 miles. I recently had some work done to the rear end transmission. When I had hit the gas when going at high speeds, there was a loud vibrating noise coming from the bottom and rear of the vehicle, and when going downhill there was a loud whistling type of sound. Initially, my mechanic, who I've been going to for the last 8 years, looked at the vehicle and gave me a worst case scenario that the whole rear end might have to be replaced. He said it could cost a grand or more. The whole rear end did not have to be replaced. My mechanic did work on the car and fixed I don't know what on the rear end.

Before my mechanic gave me the car back he said there would be a slight whistling type sound when driving. And there is a whistling type sound, every time I hit the gas going about 20mph or more. Although my vehicle does drive and sound substantially better, this sound is driving me crazy when I'm driving. My mechanic said that there is nothing that can be done without replacing the whole rear end. Is this true? Can anything else be done to minimize or eliminate this whistling type sound?