Please Help, Need Advice about Vibration at idle

I could use some good advice, I have an 03 GMC yukon xl 5.3L. I am having a vibration that can be felt in the steering wheel and seat but only at Idle and in drive. Its Not my TB, I have had the TB cleaned, Fuel injection flush and engine mounts adjusted. My RPMs is steady and the truck runs great, very smooth when driving but vibrates when I get to a stop. The vibration gets a little worst when I turn the AC on (driving me crazy). Has anyone encounter such an issue? what is the fix? the dealership said the next step would be to install a flex pipe in the exhaust system, does anyone know what that means? Please, any advice would be great.

Hi. Have you noticed the oil pressure in your engine during these shakes?? Good idea in this case to check the e eight cylinders, to know what to do...
Is it flying down & up? or it is still the same and if so - what is the pressure?
How many miles are there on the truck? R U the first owner or bought it from some one? Any other problems, like the high temperature of the runnig engine?? Please reply.