Ok, the bike shocks are different in one way.
They have a preload ring, that when you turn it, puts more force on the spring. This makes it stiffer.
On the 4runner, all it is doing (the coil spacer) is lifting the springs up. It does not put more force between them. On bike shocks there is a finite # the spring can compress/extend, the preload ring is holding it together.
You cannot compare a bike shock to coil springs. What you CAN compare it to is coilover shocks like the ones on the fjz80 (sp?).. like fox makes where the spring stops extending at a certain point. even if you put a spacer on the spring, it does nothing, unlike the 4runner.