If you still experience vibrations after going through everything, you can dive deeper into the diagnostic using a vibration meter. It may be something you can rent affordably from a specialty instrumentation shop.
The meter can measure the vibration frequency of the shimmy when you drive at the speed where it is felt the most. Working backwards, you can then identify the rotational speed which a first order or second order harmonic is occurring. You can then correspond this rotation speed with various components on the drive train. Engine, prop shaft and drive axles all turn at different rpms at the same road speed due to gearing, so you can identify which is the main culprit using this method.
