I've been doing a bit of reading around propshaft vibrations and come to the conclusion that the M5X engine set up in the sportster really isn't going to help.
Take a look at this link
http://www.therangerstation.com/tech...onangles.shtml
There's a diagram about halfway down showing the correct way to install a propshaft in terms of angles between the gearbox and diff. The Sportster set up is the worst possible. The diff is true to the chassis but the engine/gearbox is at an angle meaning that at best you'll have and angle at one end but not at the other. In the worst case (which is what I think we have) there is an angle at both ends but most importantly they don't cancel each other out and the angles are different so by the time the rotation gets to the diff, it'll be accelerating and decelerating all over the place causing a massive vibration. This is just looking at the layout from above before you even take into account any difference in height or angle between the gearbox and diff.
I'm now determined to get the engine sitting straight even though we've been told before the car is designed this way. That's moved to the top of my winter projects list.