It seems that the u-joint only gets filled separately on the 3-speed, and not the 4-speed (there's no mention of it). Doesn't the 4 speed use an open u-joint at the rear of the transmission and an open driveshaft?
From the Service Manual section for the 3-speed transmission used on 1/2 tons.
10. Remove the square head pipe plug, lubricate the universal joint bearings through the opening, and replace the pipe plug.
11. Fill the transmission with 1 1/2 pints of transmission lubricant.
The fill plug is in the actual gear case, not in the tail housing, so those are two separate operations.
