Hellish vibration!

After the big snowstorm week up here in Indiana, I've
been having lots of fun playin around in the snow with
my SVX. After backing up into a big snow drift in my
parking spot at the school parking lot, I've noticed a
pretty heavy vibration at 55 MPH and up. It doesn't
vibrate at all below 50. I got studded snow tires on
the SVX, which aren't flat (though they are about
29-31 PSI), and they were balanced when they were
put on in late December '03.

It was only recently that it started to vibrate, so I'm
thinking its either a chunk of ice that's frozen on the
inside of one of my wheels after driving up into a few
big snow drifts... or I've royally bashed the crap outta
one of my axels or something.. Anyone know what
it could be? Any help would be greatly appreciated!
Thanks!