[MagicRat]

Unstable....do you mean it leans too far in the corners, (body roll) or the steering is vague and wanders a bit?

I would suggest check out the front end first. Check and replace worn parts. Start with the sway bar links. Consider upgrading to polyurethane sway bar bushings. They are cheap and allow the bar to work more effectively. You might want to consider a thicker sway bar, too. The E350 van may have a thicker bar that would bolt in. A wrecking yard might help you there.
Then look for worn ball joints (or king pins, depending on the year of your van). The control arm bushings, tie rod ends and pitman end joints may be worn, too.