I have a 2000 Chevy Suburban with 130000 miles. At speeds over 30 or so @ 2000-2200 rpm it shudders like we are driving over a washboard road. We get no error codes and it only occurs during slight aceleration, as soon as the throttle is let off or pushed harder it stops immediately. I was told it may be the torque converter lock up so I serviced the tranny and added 2 doses of lucas additive to no avail. I have pulled the relay for the tc lock up & saw no change. Recently the exhaust started rattling so I checked it for loose hangers etc, moved the exhaust pipe back & forth as much as possible, rattle stopped & no vibration for a day then back to same vibration & rattle. I have read there is a service bulletin for an exhaust vibration but can't believe the exhaust could make the car shudder so violently, anyone have any ideas?