The last sentence of your post should give you
a clue. I do understand about the "neutral
balance" converter also. But, something is not
"right" in that converter/flexplate area. I would
check thoroughly for bends, cracks and hole
elongation in the mounting areas of the converter.
It may be possible that the stator/turbine combo
inside might be slightly out of balance. I don't
know how you drive but I would check the mounting
bolts on the transmission and crossmember just for
an extra shot of good measure. While you are under
the car, it also would not hurt to check for the
exhaust pipe clearance to any frame/torsion bar/
crossmember. Sometimes a loose pipe could vibrate
thru the cab without excessive noise and drive you nuts! More food for thought. Good luck.