I've had great luck with ruf. It's pretty standard to mix two sets of springs, making 6 or 7 leaf packs. That said, for the weight of a 3rd gen 4runner, I think you may be better off going with an aftermarket spring that will handle that kind of weight better. If your set on trying a ruf setup, maybe look into what you can do with a set of old man emu HD rear springs. Also, 7 leaf packs are the limit before you run into clearance issues between the spring and the tie rod, so I don't see having room for an add a leaf.
