That shock's purpose is to offset the other two shocks.
Unlike other truck designs, the rear axle's shocks on the Explorer are both behind the axle (bolted to the U-bolt's spring plate). When the vehicle takes a bump, the two rear shocks apply a force on the leaf spring plates at some distance X from wherever the axle would pivot from. Because distance X exists, a torque or moment arm results which would cause the pinion to raise or lower depending on the bump or dip -- which is one of the reasons why people describe the vehicle acting a little wavy.
To counter this force, the fifth shock is placed on the differential housing at some distance negative/opposite to the other two rear shocks -- countering the two rear shock's moment arm when the vehicle goes over a bump or a dip. Due to its abnormal angle, in addition to countering the other two shocks, the fifth shock also acts almost like a steering stabilizer slowing down the rate that the rear axle moves laterally/perpendicular relative to the chassis (since leaf springs allow axles to move slightly from side to side).
On other hand, other trucks have one shock behind the axle and the other in front, which counters each other during a bump or a dip, and therefore does not need a fifth shock. This is difficult to execute on the Explorer because its Spring-Underr-Axle configuration minimizes the distance between the chassis and the axle (unlike on a Ranger), not to mention the other stuff thats in front of the axle that would otherwise get in the way of a front shock (gas tank, exhaust, the body mounts behind the Explorer's rear bench seats, and so on).
Below is the Ranger rear suspension, and below that is the Explorer's rear suspension. Be aware that the fifth shock is not in the image, but you can clearly see the two shock location designs.
- data integrity people!). And now, the thread is confusing to follow, thanks to me 
