I saw a third shock on the rear of a explorer. It is sideways. Does anyone know what it's for? Sway, axle rap, torque steer?

 

That is on a much older Explorer with a live axle. Later models have IRS and do not have that shock. It is an "axle damper" designed to limit lateral axle movement. Some report no difference without it, some claim improved results after installing a new one.

 

That picture is not a live axle.

 

Looks like one to me, hung on leaf springs.

It is not IRS, it has a center diff section that looks like a Ford 8.8, an axle tube clamped to a leaf spring.....what do you think it is if it isn't a live axle?

 

It's a live axle. I still don't see what it does. How does the axle move lateral on leafs?

 

Lots of ability to move laterally on leaf springs. Some rubber bushings here, a little flex there, you can get plenty of movement. Has to be that way, otherwise you'd be breaking things all the time.

 

Shocks don't "limit" the movement of anything. If they did your ride wouldn't ride so well and you'd be replacing a lot of broken shocks. They absorb energy and convert it to heat and in so doing they "damp" motion.

 

Shocks limit suspension travel in many vehicles. The limit of full shock extension is the limit of suspension travel. I found this out with our '66 wagon when the coil spring conked me in the head after I removed a shock.

As for the lateral movement, the shock is a damper, limiting oscillation. Regular shocks do the same thing, in the manner you describe.