There are weights on the axle housing I would asssume to dampen vibration. Some Shelby owners have removed them.
That being said, and no hating on GM's as I previously owned a Corvette, the 12.96 pass is impressive simply due to the fact that the Mustang is down 111 horspower to the LS3. A GT is much lighter than a GT500 or a Camaro/Challenger. It will be interesting to see what the results are if the horsepower on them is increased for 2011.
2012 F150 5.0 S/C
Some old stuff