You my friend, are a prime candidate for a WDH. The Triton has a very long back-end. The towball is a looooong way from the back axle, and this creates a very long lever effect, lifting the front of the vehicle off its suspension. While airbags and heavy springs will prevent sag at the back axle, they will not prevent lift at the front. What you are feeling sounds to me like the van is pitching the vehicle, using the rear axle as a fulcrum. Your stiffened rear suspension is simply serving to increase this effect.
edit - by stiffened rear suspension, I mean that when the rear suspension is sitting down on the helper springs, there is very little compliance left in the system.
edit - by stiffened rear suspension, I mean that when the rear suspension is sitting down on the helper springs, there is very little compliance left in the system.