Like I said I don't think they make lowering shackles. I don't think there's room to put extended shackles on it. Remember the springs hook to the shackles higher than the shackle mounting point so to lower it you'd need longer shackles, just the opposite of cars. You might be able to put a shackle 1" longer that stock in it but is about all the room you have before it hits the bed and that wouldn't even lower it 1". Only about 1/2" at the most. Also if you did it would have the same effect as the modified hangers, would throw off the pinion angle and eventually wipe out the rearend.
The only way to do it correctly and not have problems is either lowering blocks or have the springs re-arched. I first put in aluminum lowering blocks and they broke so I took my springs to the local spring builder and had the springs re-arched. I also had him ad an extra leaf so I could haul as much as before. Still wasn't low as I wanted it so I made up some lowering blocks from 1x2 steel tubing. In the end it worked great.