Yes, the rear universal joint area needs to be filled separately from the main transmission fill. Failure to fill this area will lead to near immediate universal joint overheating and failure.
The two fluid areas do not cross over.
How do you fill the u-joint enclosure ball/bell?
Transmission fluid "migrates" to the enclosed u-joint on 1/2 ton Advance-Design trucks.Forgive these two rules of thumbs : after replacing an SM420 on a 3100:
1. warm transmission/fluid: fill the transmission up to the filler plug-hole on the upper passenger side of the SM420
2. cold transmission/fluid: fill the transmission to a thumb-length below the opening
The 1/2 ton u-joint-housing/u-joint-cover/bell slides back and forth on the outside of the torque-tube.
I clean/shine that surface so the torque tube can easily slide a little in-and-out with no/minimal leaking.
After a while, they all leak/drip a little : the old saying/joke was - if you have no leak at the torque-tube seal, you have low transmission oil.
