This sort of thing definitely a symptom of poor weight distribution, which a lot of minivans have anyway. Now that some weight has been removed that will just make it worse.
Think about it this way: over the front wheels is the engine, transmission, differential, driveshafts, etc. Since your van is front wheel drive, basically everything that makes it go is right there... that's a lot of weight. Over the rear wheels, what is there? A third row of seats, which make very little difference if no one is sitting in them, a spare tire, and if equipped a tow bar/ trailer hitch. It sounds to me like what you need is more weight over those rear wheels if you want your van to be more stable. Get a spare tire... hell get 2! mounted on some nice heavy rims and carry them in the back, that's a start anyway. Maybe get a trailer hitch, even if you're not towing a trailer, just for a little more weight.
I know people who have pickup trucks and suffer a similar issue, so they load the bed with cinder blocks, bags of sand or something heavy when the roads get slick. They say it makes a big difference. Letting just a little air out of the rear tires might help too. They'll probably wear out just as fast, but at least they'll grip better.
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