I just bought my wife a 06 cts with 58,000 miles,from a certifed dealer. i work away from home and haven't seen it or driven it yet, the problem is that she says it is making a rubbing noise from the rear end when she turns sharp. The dealer ship told her to drive it hard in curves to loosen up the rear end because the lady that owned it before her didn't drive it hard that it is a sports car and u have to loosen up the clutch plates in the rear diff does this make sence it is a auto trans. with the 3.6l v6.

 

That's quite a run-around!! CTS diffs are known for making gear noise...just not a really good design, imho.
A rubbing noise in a sharp turn sounds more like...well....something rubbing. It could be the suspension is moving enough for the tire/wheel to rub on an e-brake cable, or other component in the area. It could even be rubbing on the body....
I would suggest a visit to another dealer, or automotive repair shop for a second opinion. The posi clutches in a diff will normally make more of a grinding, jerking, or poping noise, that can be felt when turning corners.