International 9400 vs 9900

[Daytripper]

I have a 05' 9400 sleeper...the truck rides like a teeter tottering when loaded....I figured it would ride nice with the longer wheel base...shocks are all new...complete new leaf springs in both axles in the rear...is this just a characteristic of this truck? I'm looking at a 9900 series international and wonder if they ride the same(appear to be the same chassis but just upgraded cab)?