2005 V6 Vibration At Idle In Gear With A/c On

My daily driver has developed a vibration you can feel in steering wheel and seat, at a stoplight with the car in gear and A/C on. If i keep my foot on brake and increase rpm about 50 more vibration goes away. It's like at the RPM the car wants to idle at causes the car to resonate. Can RPM be raised 50 without much trouble? My other cars have an idle stop screw adjustment, those i can adjust.
Many Thanks
Richard.
 
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