Driveline Hitting underside of floor (at rear)

If I were you I would locate 2 semi-long leafs and add those the existing springpacks to firm them up a bit. Then maybe install a 0.5" or 1" lowering block to bring the stance back to your liking.

As an even more cheaper solution you could also try and add sturdy spring clamps to the leafsprings to limit springaction and axle windup.