However, you absolutely must consult your owner's manual. The exact location and type of fill plug can vary slightly depending on the specific rear axle configuration (e.g., 8.5-inch, 10-bolt, etc.) your truck has. The manual will show you a diagram and provide specific instructions.
Trying to find it without the manual may lead to you putting oil in the wrong place. There are other similar-looking plugs on the differential, and you'll want to avoid adding gear oil to anything other than the differential itself.
