Rear diff, viewed from back of truck...the threaded plug sticking out near the bottom is the drain....the one on the side of the diff facing you, mostly obscurred by the rear sway bar...is the fill plug.

For almost all diffs, you simply fill it to the lower edge of the fill hole.

The front diff looks similar...but is harder to see easily:



To be sure you have it...notice the drive shaft going into it, running from the middle of the truck, forward...and bolted in a circle at the flange....this view is from the P side middle of the truck, looking forward at the back of the front diff.

A half shaft, running to each side's wheels, comes out its sides, for further confirmation.

Use synthetic diff juice...its way more moisture resistant than the dino juice.

Good Luck!