Air bags and suspension are separate from WDHs. Do not substitute bags or suspension for a WDH or vise versa.
WDH is to transfer the weight to the front wheels. If your vehicle is designed to tow with a WDH it should always be used regardless of suspension setup. This provides the correct setup front to back, and also lessens the weight on the rear axle and tyres (check your limits)
However, that load is still distributed across both axles, the vehicle sits lower, and the suspension performance is reduced - the suspension might start to bang the bump stops over bumps or potholes, or just start that soft rocking back and forth when you stop or start, etc. Suspension is there to make the ride smooth not to take load per se. However, as mentioned earlier, there are packages available which enable extra load on the axle also, so it starts getting confusing.... But, any suspension will sag with any load, regardless of type, size, lift or capacity.
Airbags are designed to work with the suspension. The idea is measure the ride height before loading the vehicle (either contents or van). Load and then measure again, on both sides. No the fun bit.... Unload and fill the bag with air, reload, and measure. Unload and add or reduce the air, reload and measure. Keep doing so until the original ride height is reached. Then the suspension will work as designed, there will be less body roll, etc.
As it is measured on both sides, and your usually have two separate values, you can adjust to get an even ride - eg one side of the vehicle you have a fridge which is fully loaded, on the other are some sleeping bags. Naturally the fridge is heavier and will make that side sag lower than the other, affecting your handling, tyre wear and then van sway since you'll keep correcting. Ultimately, the airbags could end up with 35psi under the fridge side, and 20 under the light side to get that balance required.
As the bags work with the suspension, if your suspension has gotten on a bit and already has a permanent sag, the air bags start taking more of the load than they should and the ride will be worse, so you need to make sure the suspension setup is still in good condition. Don't spend on bags thinking that will fix your problems, if the old coils and shocks aren't as good anymore.
In regards to the Hilux vs Prado, please correct me if I'm wrong, but the Hilux has rear leaf suspension designed to take 800kg or so in the tray (over the rear axle) and the Prado has a lesser load so a different setup with the coils and shocks. You should always get more of drop in a passenger vehicle than a ute.
