My mom has a 98 GMC Burb SLT. and when she is idling the whole cars is vibrating pretty heavily. we were at a red light so we put it in park and it was still vibrating, we also stuck it in neutral and it was still vibrating. Its not a little shake either. and we can't tell if it happens while driving cause where we live its always bumpy.

 

this is kinda a shot in the dark...but you should check the condition of the motor mounts