engine vibration on 2005 Chevrolet Trailblazer

vibration at low rpm when at a stop. vibration increases when ac is on.

2 answers
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??

Roy