Vibration at 2200 RPM

If it does it in neutral:
then it has nothing to do with the chassis..... at all.
In this case it is something attached to the crankshaft, and usually ending at the convertor.
I seriously doubt that the problem originates in the tune; unless the timing is jumping back and forth.
In my lifetime I've had a couple of cracked flexplates thru one arm only, and they did Not vibrate, just noisy.

At 2200;
If it's not load related,
then it's Not ignition timing.
My professionally balanced 360 runs smooth as silk at any rpm, on timing @2240 from 46 to 56 degrees, even cruising down the road.

Rule out the fan/belt/waterpump/alternator, by flipping the belt off.
Also check the cylinder pressure (not that I think you have a compression problem.)

After that, sad to say, without documentation, IMO,
the engine will have to come out, apart,and be rebalanced.