I've been searching long enough now I ask.

I eventually want to swap to an LSD in my SA, do I need the whole rear end hub-to-hub or just the center section?

 

You can do it either way. A center section from an 81-83 GSL diff will bolt right in, and retains the drum brakes. However, the 83 uses a different driveshaft. 84-85 centers cannot be used, due to different axle size (diameter). Some say the late 83 had these larger axles, but my late 83 did not.
A complete unit from 81-85 will bolt in, but you will need to modify the brake line, and make sure you get the driveshaft to match if the rear axle assembly is from an 83-85. You will also need the park brake cables from the disc brake unit.

 

OK I want to keep my drums... so all I need is an 81-82 center section, or an '83 and drive shaft.

 

That's pretty much it. But just to throw some more information in that the early 83's still had the small driveshaft flange as it was a mid year change, AFAIK anyway.

 

Stupid little changes in first gens...