Tim is correct. It is the opening to lubricate your u-joint. By far it's the most missed service item on our trucks. From the factory, it had a square headed pipe plug. At some point someone put this fitting on you truck.

The service manual does say your u-joint needs one pint of gear oil. I would not recommend grease. I have seen u-joint failure partially caused by grease (possibly not enough).

I use a cheap small hand pump with a small hose to get gear oil to that spot one a year. I drive between 3000-4000 miles annually.


SWEET
Sergeant At Arms: Old Dominion Stovebolt Society
BUNS?!?!?!Where we're going, we don't need no buns.....
1950 GMC 450
1951 Chevy 1/2-Ton
The GreenMachine
In the Stovebolt Gallery