Tim hit the nail on the head, the fitting was used as an easy way to inject a specific amount of gear oil into the u-joint housing. This is one of the most forgotten lubrication points on these trucks. For a half-ton (and possible bigger) the shop manual calls for 1 pint of gear oil in this area.
What ever you do, do NOT use grease. I have seen what grease does inside this housing and believe me, it is not pretty.
