I have had the bad vibration at idle lately (110k mi. on the Envoy).
I bought 2 AC Delco motor mounts and so far have replaced the passenger side only. The vibration is completely gone, it idles and drives nice and smooth now. Drivers side will also get replaced since they are the same age.
The old mount is about 1/4" shorter than the new mount so I wanted to see how the mount is made and what went bad. I cut into my old mount with a hack saw and found out that the mounts are liquid filled and look like a large ball joint inside. They are not just solid rubber mounts!
Here is a picture of my factory mount cut away:
Very interesting!!!
Another tip for Trailvoys with 4wd:
I unbolted the passenger side motor mount, then jacked up the motor until the axle touched the frame. The mount still needed another 3/4" before it would come out. I then found it easy to unbolt the bracket from the engine block, remove the 2 lower bolts and just loosen the top 2 bolts. This lets you lift up the bracket and easily swap the motor mount.