Try pushing in the clutch and putting it in neutral at that speed. Does anything change?
Once the motor has been brought up to temperature for 20 minutes, cooled down and rechecked for proper torque on everything accessible, I drive the car like I use it, hard, and I only rev it hard once it is fully warmed up though, at least 15-30 minutes.
Do you have a wide band on the car? Curious if it is a flat tappet or roller cam?
Grab the driveshaft yoke and see how much play there is in your tail shaft bushing. The yoke should not have any noticeable free play. I've put new ones in and still had excessive free play with brand new yokes. Seems to be an issue that most of us just live with, and never get ride of the resultant vibrations.
