It's so much fun chasing these vibrations right. When I first got my car it was really bad between 3000-4000 RPM. I started by rebalancing tires and that helped a little. I replaced the ring and pinion and U joints and had driveshaft balanced, got a little better. Found some metal to metal contact on one of the exhaust pipe hangers and fixed that and saw an improvement. Lastly I switched out the incorrect motor mounts for OEM. The vibration got smaller and smaller in the RPM range everytime I fixed something to the point where it's now only noticed in a few 100 RPM range. But occasionally it comes back and is more noticeable, much more. I am inclined to think a lot of the issue is the 47 year old bushing in the car and the exhaust hangers not soaking up natural. I am out of ideas other than dropping the trans and checking flywheel and pilot bushing. But at this point I think it unlikely that's the issue given the intermittent nature of vibration now.