My vibration was in the rear end, only under power, but it was a newer vehicle. With the clutch in, coasting at speed, no vibration. Clutch out at the same speed, vibration, but not constant. It was a rythmic vibration, like you can get out of a U-joint going bad. All new U-joints didn't fix it. New rear bearing in the tranny didn't help. Drive shaft balance didn't help. New pinion bearings in the rear end cleared it right up.
