Stiff suspension, what to do?

My suspension is uber stiff. I have to go over speed bumps at a crawl, and can feel every little crack in the road, even the ones filled with tar. Really. I can try pushing on the bumper, but to get it to move more than a 1/4" I have to stand and jump on it, and even then it only moves maybe 2 inches. It levels out fine after that, there's no wobble or anything (obviously). If I'm changing lanes and there is a small pothole as I'm moving across, the rear end actually bounces up and slightly drifts, scary stuff. It's a 1/2 ton truck.