What should be removed to check the fluid level in a differential?

To check the fluid level in a differential, the fill plug needs to be removed. The fill plug is specifically designed for accessing the differential's oil reservoir without fully draining the fluid. When the fill plug is taken out, it allows easy access to check whether the fluid is at the appropriate level. The differential should be filled until the fluid reaches the bottom edge of this plug, indicating that it is filled to the correct level.

This method is essential because maintaining the correct fluid level is crucial for proper differential operation, ensuring lubrication, cooling, and functioning of internal components. Checking with a dipstick or removing the driveshaft is not standard practice for differentials, and spark plugs are completely unrelated to differential fluid levels as they pertain to the ignition system in an engine.