Harmonic Vibration 3500-4000 RPM

So I went for a drive with my cousin. His opinion was that there were multiple vibrations. He believes that there may be problem with the pinion angle; the diff appears to point up a hair. He's a four wheelin guy. He wanted to measure the angle and weld new perches on the rear end. But I'm thinking it's out of 76 Dart going into a 72 Scamp, how could the angle be wrong?

The driveline isn't stock. It was an aftermarket one. Maybe there's something wrong with it. Maybe it's too long. Maybe it's unbalanced.

He also thinks that there's a separate vibration coming from the motor. We did feel extra vibrations from 3000 to 4000 rpm in neutal, but it wasn't rhythmic. He thought the slight wobble on crank pully was giving me vibrations, but not the intolerable thrumming I'm feeling at speed. And when I look at the harmonic balancer, I see a change at idle. Tough to say it it's a mark on the balancer or it's wobblin' too.

He also felt that the flex plate or torque converter could be at play, but less likely.