Its a bad idea to use add a leafs, they will give a harsh drive quality and since the suspension will be stiffer to allow the truck not to sag it will decrease its mobility.
So yes you will loose flex in the rear. I'd just use the 10 leaf packs. Its spendy but worth every penny. The 11 leaf packs are too stiff. I run the 10's with a 38" spare tire, tools, air tank, spare gas tank and winch off the back with no sag.
Blocks will keep the ride the same as it is now and will lift the rear and is cheap. You might need longer shocks.
