I will assume the axle is the regular 8.8" C-clipper....same axle as used in my '97 F150: My Alldatadiy.com manual shows a capacity of 5.5 pints.
The tag is correct: Do use only a good grade of 75W140 full synthetic GL5 or better. I like Valvoline (mainly because it's readily available).
The differential is nominally full when a crooked finger stuck into and downward into the fill port can bring out a drop or two of oil on its tip.... and DEFINITELY full if the oil begins to run out of the port as you fill it.
Of course the truck will be on level ground during the fill process. (Slap me on the back of the head for that comment.)