[92snowmachine]

if the tires showed to be out of balance when respun then they were actually out of balance. this can be caused by multiple things. as simple as water in the air that inflated the tire to more complex things like shifting in the cords of the tire, or even just mounted incorrectly. water is easy to look for just unmount the tire, shifting in the cords can typically be seen by looking at the inside ribs on the tire. if the cords are shifting then the thin rubber on the inside that covers them will seem loose when you rub it. some tires (particularly bfgoodrich) actually have a particular way they want to be mounted relative to the valve stem. there will be a little circle that goes 180 out from it.
i would put money on a tire issue because no matter what else can cause vibrations when you rebalanced the tires they were out of balance. i have kept my mouth shut about these tires because many here seem to like them but i try not to sell them at work because i have had issues similar to this that was only ever solved by replacing the tires. they should swap out your tires with new ones and send the ones you have back as deffective. they may not be deffective but it would eliminate them as a possibility if you still have the vibration.