Super Chevy (dec 02)recently did a write up on vibrations with the drive train. The car in the story, I think it was a Chevelle, developed a vibration over 3000 rpm. The author checked everything. He rebuilt the tranny, ordered a new drive shaft, balanced tires, and the rear end rebuilt. The problem was still there. Turns out it was the drive shaft. The drive shaft and yoke were balanced as a unit off the car up to 2500 rpm. When he hit 3000 rpm the vibration was still there. They used a device called the MTS 4100, which allowed them to balance the drive shaft in the car. Turns out that 2500 wasn't fast enough to demonstrate the small imbalances.
1969 Camaro 350, Trickflow heads, ST-10 4spd, Hotchkis suspension, Baer SS brake kit, moser 12bolt