I ONLY found the grease fitting on the Cardan joint when I was rebuilding the entire front DS on my workbench at 100k miles. In fact, the Cardan joint was still good at 100k despite NOT being greased the entire time. This lack of maintenance was due to me and TWO dealerships NOT being able to find it with the DS installed on the truck. In fact, BOTH dealers told me that at som point Dodge stopped using the greasable cardan joint in favor of a "lifetime" cardan joint (lifetime being the life of the CJ, not the life of the truck…lol). I no longer believe this to be true, even though I used to insist on it because they said so AND I could NOT find a grease fitting on my DS to prove them wring. Now, I don't know if ALL 3rd gen trucks have the fitting, but mine certainly did, despite not being able to find it.
In any case, the small divot you see/feel is in fact the grease fitting. The hole in the center is closed with a small bearing/spring just like a zirk fitting, but if you can get a good seal between the rubber tip and the CJ, it will force grease into the fitting. It also helps to rotate the DS between grease pumps to help "coat the center ball bearing that IS what needs the grease. That being said, , it is not that all that heard to pull the front DS (mark flange orientations first) and then clean it up and do the work on a workbench. THAT is when I found mine and was able to work the grease into the cardan joint extremely thoroughly…I then CLEARLY marked the fitting with a die grinder and some dayglo paint. so next time I will be able to find and grease it on the truck.