I had this problem with my R32. Is it a manual conversion? Some guys mix and match driveshafts/weld them up themselves. Mine was about 50mm to short and hung out of the back of the gearbox which got interesting at speed, i ended up getting a custom one made up. it would pay to get your driveshaft balance checked.
It could be many things,
- Driveshaft uni joints
- Diff bearings
Has the diff been shimmed?, this can chew out the diff carrier and cause slop in the assembly. get under your car and give the halfshafts a good shake, if there is a bit of slop that could cause it.