Why is it, if they are not built very strong that they have a much higher towing capacity than the manuals?

What you talking about is basically a system balance, the shaft can be balanced at the factory or at a drive shaft place but the auto and diff may not be perfectly balanced, a system balance is when the whole driveline is span up and completely balanced. BA onwards have this standard from the factory. this is why the yellow dots on either end of the shaft MUST line up to the original position when drive shaft is removed.

I just picked up my XG ute ... and I am getting some vibration in 5th between 100-110km/h ... not in 4th though.

And as Jase told me ... just do 120km/h instead ... hee hee.

I might go an get the tailshaft balanced anyway ... doesn't sound too expensive by what everyone has mentioned.

What's more annoying is the metal button clip for the tonneau cover right behind the window ... ticking away in the wind.