Final answer:
A harsh ride in a leaf spring suspension could be caused by seized front spring shackle pins or a broken leaf spring, as they hamper the suspension's ability to absorb road conditions.
Step-by-step explanation:
When diagnosing a leaf spring suspension complaint of harsh ride, several factors could be at play. A seized front spring shackle pin could prevent the leaf spring from moving as designed, resulting in a rougher ride. Similarly, a broken leaf spring would affect the vehicle's ability to absorb road irregularities, also causing a harsh ride. While under-inflated tires and inadequate fifth-wheel lubrication can affect ride quality, they are less likely to produce a harsh ride specifically due to suspension issues. Therefore, the most likely causes are either seized spring shackle pins or a broken leaf spring.