John,
I think what is being referred to regarding the wheel being out of balance is probably due to uneven ice buildup on the wheel. Check out my post under "NNG La Veta Snow" of 2-8-08 about my adventure with the UP rotary in Wyoming. With the wheel out of balance due to ice buildup those rotaries will literally shake themselves apart - even the relatively "modern" ones. Probably why most truly modern snow removal machines are two stage - an auger to collect the snow and a blower to expel it. A rotary snowplow is still a sight to see in operation, though.
Phil