What happens when you go over a speed bump at an angle (about 45 degrees to the bump) at slow speed? Does the chassis really start to rock side to side? That's a sign of inadequate shock damping. If you install a rear sway bar, under these conditions, rocking will get worse!
How does the van react when a truck passes you on the freeway? Do you get an excessive side push or just a slight push? I have to compensate with steering somewhat, but it's not a white knuckle situation. If it's excessive, then you can use a rear sway bar, otherwise that's not your problem. BTW, if your tire sidewalls aren't strong enough you'll get a similar excessive side push.
Naio wrote:
He reminded me of something I had forgotten to tell y'all, which is the van's tendency to set up an oscillation after it goes over a bump or a dip. It feels like the road goes up and down and up and down... but really it was just one bump and the van doing the rest on its own. This is with new, stiff shocks. It's especially bad if the bump is hit while cornering.