Gday DOM...
Its got slipper springs mate you may have wear on top of the sliding section...Would be a bad move to load these up more than they are...they just grind right through your chassis rails (make sure you check em) Therea are trailer parts out there that will rate to 1200kgs that cost less than $200 for a full new kit.
If you go with just inserting leafs then make sure you reinforce the slipping section at the other end...if it wears through your gonna be up for more than a new spring
I chopped a set off and old hilux chassis and retrofitted them over a few weekends... shocks and all