Vibration while in gear

[H.USA]

Hello, I have a 2007 trailblazer with lot of miles on it. I'm experiencing a rough vibration when I come to a complete stop but it goes away when I'm driving, in Neutral, Park or reverse. Sometimes it doesn't occur at all. It looks like it's not related to the RPM since it's not moving when the vibration is occurring. I also tried stepping on the gas a little bit(with brakes engaged) but it won't go away.  If engine or trans. mounts are causing this why this isn't happening every time ? And If it's definitely Mounts is there a test to Identify which ones? Could this be Throttle body control unit or MAP sensor? Thanks a lot!