vibration at low rpm when at a stop. vibration increases when ac is on.
engine vibration on 2005 Chevrolet Trailblazer
Motor mounts are common, but could be something as simple as cleaning the throttle body out. When its dirty, the vehicle idles a bit lower than normal, even worse when the A/C is on. This can exaggerate normal vibrations.
check motor mounts. how does it run?? are you up to date on all your services??