Below is a picture of the XL-320 servo casing opened up. The pinion is the small gear on the right which is above the servo motor.
Next we have pictures of different types of pinions from other servos.
It looks like the plastic pinion gear in the XL-320 maybe causing a few issue. See my video below.
It looks like the plastic pinion gear gets deformed overtime by heat and use. This then causes binding of the servo when both under power and not under power.
You can tell when the servo is starting to have issues when you get a servo red light, the servo stops moving or when it stops moving when the servo is not powered up.
I have some good news though. Dr. Chi Thai has posted a picture of the new version of the XL-320 with a brass pinion.
Based on his comments below. The new brass pinion fixes the issue and improves the movement of the servo!
"I just replaced all the leg actuators with the new XL-320s with metallic pinion gear. The arms and head are still using the XL-320s with plastic pinion gears. In the video, you can hear actuator #5 whining a bit, but the legs can be manipulated quite smoothly now."