Another vote for Bilsteins. After a lot of research they turned out to be the best bang for the buck, by far. Shock Warehouse sold me a set for just under $400 - no tax or freight. After talking to them and Bilstein I ordered the heavy duty version for my E-350.
They are a PITA to install so I had it done at the local truck center for $287. Two guys wrestled with them for nearly 3 hours so it was well worth it to me. As expected the improvement was off the charts and I could not be more pleased. Like Koni's they come with a lifetime warranty.
:C