So in terms of what to do to the rear its dependent on how much you want to spend / what you want to get out of it. Add a Leaf sounds great for this sort of thing, but my friend says that directly after he put those on his his rear suspension started to suck. So in terms of just gaining a couple inches I would just swap in a couple extra toyota leafs into the spring pack.
Shackle length is a debatable thing amongst us interweb guys you'll hear a load of different things. Fact is its not super important. There is a geometrically perfect length of shackle, but you have to factor in mounting location which requires you to cut and reweld the shackle mount. In the end for someone not doing anything super big as long as your around or near 10% of your curved spring length your fine. (measure this center to center from the eyes.)
Now if your planning on spending a little money a bit more than 200$ could net you a set of old man emu springs. You would probably have to call or e-mail them so that you get the direct bolt on length springs with the amount of lift you prefer, but here's a link to those springs:
CS009R HILUX 4RNR REAR (OME17) - OME Dakar Leaf Springs - Suspension Systems - Online Store - OldManEmu.com
Someone might suggest using lift blocks under your springs, I and a lot of other people would consider this to be a bad thing to do and I would suggest you ignore that particular suggestion.
Happy Trails!