This is the reason I couldn't find anything about it when I searched '4th gear vibration.' As you said, It is actually when the Torque Converter locks up around 70kph, the vibration starts. 50kmph drives smoothly.
Upon a little more investigation, it must be over drive not 4th gear and it seems this is a common problem usually caused by a Torque converter failing. It's not bothering me too much ATM but I'll do the drive shaft uni joints and flush the tranny a couple of times and cross my fingers...
Rightio. I have an automatic (a440f) sitting behind a 12ht. When I casually accelerate up to 70km/h, it changes into 4th gear. The second it changes, I get this nasty fast vibration coming from the rear of the vehicle. I can make this vibration continue if I hold the revs and speed. Although...