I have two wagons and drive them year round...lots of snow and ice in the winter. The IPD sway bar, all uretahne bushings help, but the girls still like to wiggle thier butts, and it is very easy to rotate on ice (and sand!). The overload coils also made a positive improvement, less leaning in back. I think a rear sway would really make a difference, but have not found one made specifically for the 220. Hancook makes a pretty stiff sidewall that I have liked for summer driving, and michelin makes one too although it is more $$. You mentioned that it was your work truck, having a full load in the back changes the handling dramatically of these cars. On mine the cornering attributes are more accurate than the fuel guage :).
When you replaced the bushings did you have all four wheels aligned? having the rear off by just a little will make an impact (more wagging).
Otherwise, I recommend you take it out on the ice and practice when no other cars are around to avoid over correcting (spinning) and try down shifting on the ice when turning, you can steer the rear with the gas pedal if you don't have an LSD. :)

Good luck and enjoy.

-Erik