Take the inspection cover off the transmission and if the motor has internal balancing then turn the motor over by hand and look for the rectangular weights that could be on the convertor usually near the drain hole if applicable to that convertor. You have nothing to loose by knocking the weights off and seeing if at idle the vibration goes away.

Do you know what flex plate is inbetween the motor and transmission. It could be a factory plate or it could be a balanced flex plate for a 318 or 360 that would also toss the weight off.