If that's the GM 6.5, two things come to mind:
1). There is a device known as the PMD (Pump Mounted Driver) that was mounted on the side of the pump when delivered. If yours is still on the pump, look into a PMD relocation kit. The PMD was subject to high failure rate when left mounted on the pump. It's possible this has already been done, but good to check to make sure.
2). Check, or have checked, the harmonic balancer. They're known to fail, and as they fail the engine experiences more and more vibration. Crankshaft failure can result.
Other than that I don't know anything about the bus part. I Don't have a bus (yet), but do have a 1-ton Chevrolet 6.5.