I bought my 318 Dart and had the same problem. I checked the converter and found an external weight. 318's are internally balanced. I changed to a 2800 stall converter and the vibration went away. Taking a weight off a converter does not make it neutrally balanced any more than taking the weights off your wheels neutrally balances them. The weight balances any manufacturing or machining issues that were already there. Now it is unbalanced. I would certainly consider a new converter, and since you are already there, go with a nice stall.