Sounds like nothing more that springs.
Without weight they are sitting relaxed, but can't support the weight anymore.
Give Atlas or National Spring a call. They'll build you a set of springs that are setup exactly how you want....length, curve, spring rate, etc.
Surprisingly, they are not that expensive.
Don't let a spring shop "rearch" them. All this will do is make the ride height higher and make them stiff, but they will fail eventually.