rear gears

I have an 18 manual with the 3.55 LSD. The instrumented tests of the 2018 Mustangs you find on the web and in the magazines are done with the PP Mustangs that have 3.73 Torsen rears. I want shorter gears like the PP, but even shorter. I'm actually looking at installing the 4.09s since they...

I have been experiencing rumbling & vibration between 50 & 70 mph, which kind of felt like the wheels were out of balance. After rebalancing them twice, the dealer even tried a different set of wheels & tires - no luck. They now believe that the drive shaft needs replacing - at 1100 miles...