Front end Shimmy at about 50 mph. Transmission torque converter?

What is your question?

1992 Plymouth Grand Voyager with 135K miles on 3.3 liter 4 speed (Ovr. Drv.) At about 40 mph is starts to slightly shimmy in the front end and at 50 mph it is the most extreme, then will settle down at about 55mph. Had the front end aligned and tires balance checked. The mechanic says they've run into the same thing and it turns out to be either the Tranny lock-in solenoid or the torque converter. If it's the lock-in solenoid, is that one of the solenoids in the solenoid Block (mounter outside of the tranny? If so, I can replace that. Is there a way to determine for sure which it is before having a shop tear it apart? Is there an easy way to determine internal transmission problems yourself? Thanks. Joe