There are lots of larger trucks that don't use shocks in the rear. There is enough "spring" in the pack that it's movement per pound is not comparable to a front spring, and there is much higher friction in the leaf spring to eliminate the need.
By virtue of their being there, it's safe to assume that the springs used in the front suspension of the truck do not provide enough self dampening.
Installing the front springs in the rear, they will still have more squish per pound of force, and I could only assume this will not add anything to their ability to self dampen.
I'd think you'd be a lot better off with them...