U-joint lubrication

Is there a special grease-gun fitting that will fit down inside the shoulders of the drive-shaft universal joints so that I can reach the zerk fittings to lube the u-joint? I don't really want to take out the u-joint just to grease it. Is greasing THOSE particular fittings necessary? I have my own grease-gun, but the fitting on the end is too wide by just a millimeter or 2 to fit in the space where the zerk is.

There is a tube adapter that can be purchased that slips easily between the knuckles of the joint. Only catch is it doesn't lock onto the zerk. You have to hold it tight to the end of it.