Hey ihanberg,
It is all too common for 4Runners to sag in the rear. Toyota goofed on it. Even with the 2nd gen with coil rears, they still sag.
Sure the easiest way to level it out is to just add a block in the rear, but the spring rate is higher as its compressed and the shackle angle is changed, also effecting the pinion angle too, which gets worse as the suspension compresses.
I don't know anything about the measurements or spring rates, but you'll probably be better looking around for springs from another 4Runner or 4WD pickup that have less miles on them. Aftermarket springs are usually more arched for a higher lift, and add-a-leafs will just make the ride worse..
Good luck
BigMike