Ok, this is something that has been bothering me for years now.
the car has major slop in its driveline, combined with alittle rough cruise map, this makes for a wery uncomfortable ride. this has gotten worse over the yearsm and i have no idea what causes it.
my engine mounts are 100% solid race aluminium mounts, the diff was changed with no improvement, the diffmounts are solid Delrin. i have a solidmounted brace on my gearbox and the PPF has no cracks in it. everything on this car has been changed at one point (so its not a flywheel/clutch issue) the only things left are the axels, the driveshaft and the transmission.

has anyone dealt with this problem before?

 

nothing? :S

 

take your driveshaft off and check the u joint connecting the Yolk to the driveshaft and rear u joint to the diff. if those look great than, lastly it would be your axle's. but honestly, I haven't seen any bad FD axle to cause vibation yet. it's usually the Yolk or the u joints.