Final answer:
Extended operation with the differential lock engaged is the primary cause of scored and discolored differential side gear thrust washers due to constant pressure and heat from a lack of relative movement between the differential gears.
Step-by-step explanation:
The discolored and scored differential side gear thrust washers are typically caused by extended operation with the differential lock engaged. When the differential lock is engaged for prolonged periods, it prevents the differential gears from rotating relative to each other, as they would normally do to accommodate the different turning rates of the wheels on turns. The constant pressure and heat generated from this lack of movement can lead to scoring, overheating, and eventual discoloration of the thrust washers.
An incorrect gearset backlash or an incorrect gearset contact pattern can also cause these washers to wear prematurely. However, these conditions would generally result in a more consistent wear pattern, as opposed to scoring and discoloration typically associated with overuse of the differential lock or extended wheel spinout. While wheel spinouts can generate heat and stress on the differential components, it is more sporadic and less likely to cause uniform wear or discoloration of the thrust washers.