Sikday,
Matt is correct, check out the front end, (I did with mine when I got it) but I will also confirm your problems, I have a Quigley system on a 2006 E350. My van does the same thing, but hasn't gotten worse in snow or on ice. We've had ours now for two winters here in Utah with no issues. But I have to agree, its a little unnerving at first, you need to learn how to drive them correctly. So much so that on our way home from picking up our "new" van my wife was driving & I fallowing behind her, I ended up calling her to ask why she was all over the road, her reply was "the van is darting from side to side" "I'm doing my best but there has to be something wrong with the alinement or tires" so when we got home I check everything out. Then I changed the tires "same as you" then took it to several shops having them check it out, then even called Quigley (NO HELP), everyone said pretty much the same thing, everything is in good working order. So that was about a year ago and we have put on another 14000 miles since then an all I can tell you as we've learned to drive it the way it is... I've also learned that with the Quigley system, the rake will need to be changed and can be with some work which in theory should "calm" the steering down, this is compared to other systems that are out there. But I'm still working on that myself..
Let me know if you find something different out.. Best of luck