Modraus

I have the same problem as the OP. I took it to the dealer and they did the three-way catalytic converter settling procedure. It is slightly better but I still get a significant vibration during afm around 35 mph and in the range of 1200-1400rpm. Has anyone had this issue and had the dealership correct it? They are now saying they drove others on the lot and it is normal according to gmc. I do not feel like it's normal. It is no small vibration. I can hear and feel it in every part of the cab.