I would have the front and rear sway bars checked to see if they are aftermarket. Someone may have been trying to improve the handling and beefed these up too much. You will get a stiffer more harsh ride with heavy sway bars but thats the trade off. Since you already have experience with a 2500 pickup you know that a heavy truck rides smoother when loaded close to its rating and the E450 has some pretty beefy springs. If you get it weighed the way you plan to travel and its light in the back you can have airbags installed and have a leaf or 2 removed, if you are near or over the limit or the springs are weak from sitting loaded and the last leaf is doing any work airbags will lift the rear a bit and give you a cushion. Many times that last leaf is an overload protector and is very very stiff. A good truck suspension shop could tell you what it needs but it must be loaded the way you travel when you take it in.