Oh, you said in the original thread post that it was less pronounced with the wheels on the ground, at speed. And that it can be felt in neutral.
As for your engine balance, go through all your documents and see if it was balanced. You’d surely know if you bought a balance job or not. It’s at least a couple hundred $. And with a good balance, they want to also have the flywheel/flex plate, balancer, and sometimes the torque converter. Not just a crank and block.
it would be my assumption that if the engine was not balanced, it would vibrate and has a resonance at that specific rpm range. I had a 360 with KB pistons, no balance and it would have a certain rpm spot it would resonate at.
