I had a severe vibration accelerating around 3000 to 3600 if memory serves me correct. Turns out that the driveshaft universals were out of alignment approx. 10 degrees. Where the front and rear sections meet at the the middle rubber joint is where the misalignment was. A new shaft and mucho dinero cured that vibe. But I still have a lesser vibe between 2600 and 3100. Have weighed rods, and pistons, pins and rings. chuck