The rear suspension of the wagon uses nitrogen/oil accumulators as part of the self leveling system. The nitrogen provides an air spring kind of functionality. Over time the nitrogen leaks out and you loose the springy-ness, resulting in the hard ride you are now experiencing. The fix is to replace the accumulators (there are two - one connected to each self leveling strut in the back).

It's medium difficulty for a DIY mechanic. No idea on the pricing of the parts, it's been too long since I shopped for these. Probably have them listed here on Peachparts.