Sometimes the bearing/check valve is simply stuck from non use or corrosion. Whether stuck or clogged often poking with a small scribe or pick will allow grease to flow.
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I had a Bill Davis (awesome guy). Great Basin Rovers shaft. I had it greased every oil change, but the shops that greased it neglected the centering ball. It started squeaking at 15k miles maybe 20k, and I got a warranty replacement from Bill. I didn't realize the c ball was flush mount nipple and stressed the importance to each shop of greasing it and again chirping at about 20k miles. I got a centering ball kit from Bill and had a local shop replace it but the yoke was too worn out. I would 100% recommend Great Basin rover shaft, but I went with BPUtah for my third drive shaft. Now I understand how to explain to any shop how to grease the centering ball, and I will get one of those needle fittings, and keep it in my D2.
Without grease, the centering ball will blow out at about 20k miles. Subtle symptoms will start around 12k miles... subtle vibration, maybe a squeak here and there.

 

Ok thank you guys. Since we did the HG ourselves, im trying to tacke all the maintenance work on my discos. I went to get it greased and oil changed at a shop near by.
NOTE, im stupid and did not realize that the grease gun end had to be locked by loosening the end, fit it in and lock it in place before pumping the grease. haha! Another lesson learned.