Here is a pic of a bad mount. As you can see the rubber is squished down (doesn't always but this was how mine was), but the rubber also separates from the metal allowing the fluid in them to seep and leak out. Without the fluid in there to dampen the vibrations it will cause the car to vibrate. This is the one of the number 1 causes to most vibration problems. You can't see the problem till you take the mount off and look at it. You can however feel it if you stick you hand under the mount and feel if the rubber is separated from the metal of the mount. Like AMC said go for a ford mount the aftermarket ones fail much quicker.