2007 Chevrolet Express drivetrain vibration

The van is a 3500, 6.0, with auto transmission. There is a momentary vibration similar to driving on a rumble strip or the lugging of an engine with a manual transmission vehicle. This will not occur under hard acceleration from a stop but during the normal up shift sequence, occasionally at low speed but especially at cruising speed between 60 to 70 mph as the load, incline and speed vary. Usually beginning a high speed pass will produce this vibration as well. I have had the engine mounts and drivetrain checked out (u-joints, carrier bearing, shaft) and the tranny serviced. Consideration was also given to suspension and wheels. After reading a bit on torque converters, can my problem be originating with that? I operate an airport shuttle service and will replace this vehicle in the near future but would prefer to get a bit more service life from it. It has been well maintained, but if it is torque converter related I am concerned with a complete part failure while the vehicle is in service. Can I get more reliable use from the van as is, or weigh out my options for immediate repair or vehicle replacement?