You can get yourself a needle fitting if you can't get the standard size grease gun
fitting to go on due to space restrictions.
I have the one shown below that I bought about 20 years ago when I got my 1985 Jeep, and I saw that all the
standard size grease gun ends would not fit into the U-joints.
This needle adapter just snaps onto any standard grease gun because it has a zerk on one side (shown on left side of the picture)
and then you just press the other side into the center of the zerk in your U-joint to depress the ball bearing
and create the seal, and then pump in the grease.
You have to hold the needle end of the adapter onto the U-joint zerk as you pump the grease gun, so a gun that can be operated with one hand is needed.
When your done, just pop the needle adapter off the grease gun, and finish doing the rest of
the Jeep as normal. Zip Zip...
Oh, and these things only cost about 7 or 8 bucks. You should be able to get one
at any Pep Boys or other car place.