Magnum 5.9L vibration

I ran a stock bottom end 5.9 Magnum, and had to run the stock 5.9 Externally balanced harmonic balancer, which it sounds like you are, and the stock 5.9 Magnum flex plate with one of the torque converter bolt holes slightly elongated to mate with the converter. Not sure how you balanced a non-5.9 Magnum flex plate, but I'd suspect that.

You must also run a neutrally balanced torque converter with the above mentioned items. Some of the factory LA converters were externally balanced too I believe.