Torque convertor balance questions

I know a guy trying to find a driveline vibration and he is running out of ideas. He has a new rear axle, new mounts, tci transmission and a new driveshaft but hasn't found the problem yet. So I was wondering how noticeable are the vibrations if you put a neutral balance converter behind an externally balanced engine? Or if you use an external balance convertor with a weighted flex plate? I was wondering if he may have some mismatched parts and if this could potentially be part of the problem/