Final answer:
The residual trace of oil on the shafts of a vehicle's rear shock absorbers indicates that the shock absorbers are defective and need to be replaced.
Step-by-step explanation:
A residual trace of oil on the shafts of a vehicle's rear shock absorbers suggests that the shock absorbers are defective.
Shock absorbers are designed to dampen the motion of the vehicle by dissipating energy.
When they are working properly, they reduce bouncing and oscillation of the car. However, if the shock absorbers are faulty, the car may experience bouncing and oscillation, which can be observed when going over bumps or after stopping.
In this case, the presence of the oil residue indicates that the shock absorbers are not effectively damping the motion, so they should be replaced.