Shake rattle and roll

You didn't say what year your Magnum engine or flexplate is, but the first couple years of 5.9s used a neutral flexplate and an externally balanced convertor, later years went to an externally balanced flexplate and a neutral convertor. I'm not sure what the cutoff date was, '94-'95? maybe others can chime in on that. I've personally only had '97 and up, but others swear that is how they did the first ones.
So, make sure your convertor has the balance weights on it, or you'll never get it sorted out.