Not to be a smart-a$$, are the tire pressures equal? Or perhaps you have a failed bushing?

Yes, I have a 400 and they came with the performance 4-leaf springs. The spring rate is 126-130 in/lbs, whereas the 5-leaf is rated lower at 89 in/lbs. I just installed a set from General Spring [in Kansas] - they have nice selection and competitive prices. I'm waiting for a part, so I haven't driven it yet...