Voila! ... problem solved.. kind of.
I had a few moments before the start of the grand prix to tear the drivetrain apart, in went a new shaft (the "old" one was still very new - still true..), new pins, rebuilt the spur + layshaft assembly, re shimmed front and rear diffs...
Chucked a pack in and Bam. the shaft vibrated just as badly, and scratched itself up! (BOo HOoo..)
so i redid it, but swapped the front diff to a one-way and not it's smooth. I checked the outdrive on my spool and surely enough, they're pretty worn. So, all a case of the fron diff outdrives? perhaps.. i think it's just that the spool is so harsh. My cvd's are still pretty devoid of wear..(TiR units). so i guess the vibration is being passed on through the spool?..
anyhoo.. i'll just go back to running the one-way. (I prefer the one-way but the spool is nice too. It's just so stable..)