A 2003 E450 has the poorly engineered and under-rated front stabilizer bar. The rear stabilizer bar is good but under-rated for the poster's particular application. Given his complaints, I advise to replace both front and rear bars with affordable heavy duty Helwig brand versions. They are fairly simple to install for a do-it-yourselfer.
New front & rear heavy duty stabilizer bars along with a front wheel alignment and the right tire pressure, all in combination will make a noticeable improvement. But I fear you will have to go yet another step and replace your shocks with heavy duty Bilsteins, and maybe adding a rear trac bar.
BUT first make sure your ball joints are not worn.