All aftermarket shifter have you the factory rubber insulator and this could be why you are feeling the vibration.
This is the explanation that Steeda give you: The vibrations are always present, but are hidden by rubber isolators in the original shifter and shift handle. The isolators which absorb the noise are also partly responsible for the sloppy, rubbery feeling of the stock shifter, so it is necessary to eliminate them for a performance shifter.
Mustang Monthly Jan 2015- Feature
5.0 Mustang & Super Fords July 2005