My dual cab hilux used to sag when fully loaded. Pedders quoted me around $700 12 mths ago to add another leaf to the springs. I removed and replaced the springs myself and took them to wholesale suspension in Penrith who did the job for $350. Problem solved.

Hi David,
Not sure if you've had a response, so hope this is of interest.
We run a D22 and tow a Tvan, and after a little experimentation find the addition of Koni shockers all round and installation of Polyairs (airbag in a coil spring) to the rear satisfactory. Standard springs/torsion bars retained.
That configuration gives ability to adjust air pressure in rear to cater for unladen or laden, and to me seems to also do a good job in the bush.
We only need to run 10psi in the Polyairs to retain standard ride height (when laden).
Rgds
Rob.