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If you have a coil spring and you force the coils closer together with a spacer (or in the case of motorcycle or high end racing shocks/springs, an adjuster) you make a significant difference in the length and stiffness of the spring.

Not true with the rear coil springs. On the motorcycle, you have a coil over type suspension which do exactly what you discribe. You are squeezing the spring further as you add spacer in the coil-over suspension.
The rear coil suspension of the 4 runner is a bit difference. The truck actually "floats" on the coil springs. When you add the spacer, all you're doing is increase the distance between the frame and coil springs. The springs do not compressed any more than before.

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