2005 Chevy Avalanche Vibration

If it vibrates at idle in gear I would rule out driveshaft and axle as that needs to turn to vibrate. But your harmonic balancer could have broken the rubber and moved. An easy way to check is to put a chalk mark on it on both pieces then give it a quick rev and see if it moves. Also the flex plate could be cracked, the torque converter bolts loose or a weight could have fallen off the converter. Unless the trans has been worked on recently and the converter has been changed as it may bbe the wrong one.