I've experienced it too, on a bad stretch of road. The road looked like a series of 8" high whoops, about 4 of them in a row bending around a curve spread about 3 feet apart. It made the truck's rear axle hop and slightly oversteer, but nothing serious. I shudder to think what it would have been like in my '98 ZR2 Blazer, that leaf spring setup hopped like a mo fo.
Its just the nature of a solid axle to hop on occasion. Progressive rate springs and different shock valving can reduce it significantly.
I'm curious. what year and model is your 4Runner? What size tires do you have and what psi do you run?