What are the symptons of a broken transmission mount on an AT?

 

Simple, under a "power load" (ex: heavy throttle, power braking) you will feel a "thud" in the center of the car where your shift tunnel/console is located. If it is broken, you WILL know it. If you just have a vibration, that could be due to a number of reasons, unbalanced tires, incorrect tire pressures, drive shaft issues (balance or u-joints), bad alignment, brake rotors warped, and so on. Even if your trans mount was just cracked, that would not cause a vibration unless under full throttle or a heavy power load. Good luck