My Lord, the ONLY thing in a "U" joint that needs lubricant is the 4 little sleeves/cups, that slip onto the 4 machined down ends of the "U" joint CROSS!! GREASE WON'T work into that extremely close fitting clearance! BESIDES that, the sleeves just move a minute amount of degrees, back & forth on the CROSS. The GREASE just gets "WHIRRED" out in the BIG cavity, BUT it probably would prevent RUST, if you ran your car into the RIVER.
The grease fitting was designed JUST for where the torque tube bell moves up & down few degrees.
"MAYBE" the filling the whole innards with grease was contrived by a Grease Company, to BOOST GREASE SALES??
A friend who has FOREVER been a proponent of the grease "THING", just BROKE his U_JOINT, it DIDN'T wear out cross or cups, just BROKE the CAGE, go figger?? A new "U" joint, installed with just 4 squirts of motor oil, would probably last for 37 1/2 years!
Grease is meant to be used, SPARINGLY, in the proper places. I've seen some that stuff the WHOLE front hubs FULL, with wheel bearing grease, "Maybe" they were going to see if they could launch their car into the RIVER, ALSO! That practice would be OK for a BOAT trailer, or an AMPHICAR which uses its' FRONT WHEELS, for RUDDERS.
Bill W.