Loose motor mounts.
There are many reasons why your car might vibrate while it is idling. It could be running too hard for example.
You might need an alignment.
It is idling. This is hard for an engine to do smoothly.
Possibly due to engine mount
Check the distibutor model and pickup coil .
As the vehicle sits idling the driveshaft is not rotating.
My guess would be to look at the motor mounts to see if they are loose or broken.
Yes, when idling and driving.
Probably wheel bearings or wheels are unbalanced
It depends on how long you are idling, or how fast you are going. If you know you are going to be idling for a long time, it's best just to put your car in park.
Car emissions can be reduced by reducing idling time and by driving at a moderate highway speed when possible. Driving too fast and idling are both wasteful in terms of fuel, and increase the vehicle's emissions.
It's a 1997 Tahoe