G'day ashh,
I just bought a BT-50 single cab, and I carry a lot of weight.
There is a product that is very simple and works.
It is a called a "Load Lift" and is basically a heavy half leaf spring that is adjustable. It lifts the rear of the leaf springs, prevents sagging, maximises load capacity, improves steering/tyre wear and helps prevent body roll... blah, blah, blah! The rear of the ute will sit at the same height as if it were unloaded.
I got it fitted to my ute after seeing it installed on another. I think his name is John (business owner), he comes to you and fits them on the spot. Cost was $380 all up. He also has Big Bore shocks to compliment the springs, but they wearn't needed on mine. His business is called CVSS (Commercial Vehicle Suspension Specialists).
Ph.0412 592 333 or 1300 780 874
www.cvss.com.au
I don't work for this guy or stand in line to receive any spotters fees. I am just really impressed with the products and service. Hope this helps in some way.