2004 Chevy Trailblazer 4.2L. Engine vibration.

What makes your mounts "good"? They are 15 years old, and at the very least, the rubber dampness are likely dry rotted and not doing much of a job dampening anything. The reason things happen your air conditioning compressor engaged and then not when disengaged has to do with the harmonics of the engine at different speeds and loads. If there is an engine performance issue at all, it will be accentuated with the air conditioning compressor engaging. That said, I would look both at engine performance components, and also look even closer at the engine and transmission mounts. As a side note -- you have another question about a clunk - mount issue can do that also, as food for thought.