833 OD vibration

I checked drive line runout in 3 places to verify it was not bent. All of my measurements were less then .020. I also checked it at the tail shaft right at the seal. I figured that was good way to verify the main shaft wasn't bent. I tried adding a clamp to the drive shaft to check the balance, but that just gave me a vibration at 1500 rpm in addition to the 2500 rpm vibration.