Sometimes while at idle, there will be a vibration felt in the seat & floorboards & also a vibration felt in the seat, floorboards, & the rear view mirror glass will be vibrant from 55MPH & up.
I have all 4 new tires(same size & brand) installed, rotated & balanced, front end alignment done after the new tires were put on. I have installed new shocks on front & rear. New front wheel bearings & seals installed, along with new pads & rotors.
New ujoints, new transmission mount installed.
What am I missing?

 

If vibration at idle I'd check for worn engine mounts.

 

It's hit or miss on vibration at idle.
You reckon a vacuum leak would cause this?

 

Anything that can cause a misfire can certainly contribute to a vibrating motor.