Have you looked into airbags? Not the ones that make the Vibe hop on 3 wheels while you cruise the strip, but the ones made for just this type of situation? I know Firestone makes some and I'm sure there are others. Less expensive than I would expect, too.
If you really wanna go cheap, you purchase spring spacers. These are large, stiff rubber spacers made to go between a couple of the coils on a spring. This raises height and spring stiffness a bit.
That's all I got.
NO! Wait. You can make everything hanging on your VIbe out of carbon fiber.