Mustang have, and have always had, a solid rear axle. Mom-mobiles like the Accord had a fully independent double wishbone rear suspension 18 years ago. Any car with pretensions to be a sports car sure as hell should not have a solid rear axle in 2009.
Solid rear axles are much more likely to unsettle the entire rear end of the car when a single wheel hits a bump or pothole in a lateral acceleration situation.