1964 dodge dart Vibration

Here is a possibility the engine has been replaced with a 68 and up engine without installing a bushing in the rear of the crankshaft. 67 and prior torque converters have a smaller snout. These bushings are available from a number of sources such as Torqueflite Patty on ebay. I would exhaust all other possibilities first before pulling your engine or transmission to check for this. Is this a slant six car ?