Welcome to the world of truck driving...
For the record, I drive a double cab pick-up with a light rear end, and
yes it is leaf sprung. It rides firm, jiggly and rattles my kidneys out,
but I like the way a truck rides, ever sat in an unladen 5000kg truck -
Mitsubishi Canter, Hicom Perkasa and the like ?
About the rear leaf spring suspension of yours, there are many factors
contributing to the stiff ride. Make sure the shock absorbers are not the
heavy-duty ones. If you do not tow or carry heavy loads, OEM shockies are
the way to go.
Secondly, are your tires old and age harden ? Try replacing them with
Highway-rated rubbers. Not All-Terrain or even Mud Terrain types.
Next, is to try servicing the rear leaf spring set. Leaf springs are
known to rust and squeek and then give a stiff ride. Furthermore they are
not seperated with teflon pads, the leaves actually rusted and welded
together, Not the case with newer designs (peek into Mazda Fighters/Ford
Rangers` rear springs) and you will know what I mean.
Removing some of the leaves will create a "sagging butt" phenomenon.
Try replacing them with a mono leaf spring (parabolic leaf spring) and
you will be happier then.
If all fails, just live with it because it will never ride like a Harrier
240G Premium L package

My 0.02 sen..