I finally made some progress with this issue. Even though the drive shaft was balanced twice, I decided to try and balance it myself. I installed a hose clamp in 12 different positions on the shaft and ran it up to speed. I found one spot where the added weight reduced my vibration. Im not sure if its because the shaft is still out of balance or if im compensating for a balance issue elsewhere in the drivetrain. either way the added weight is helping.