Where to find a decent rear diff in SoCal?

I would find out exactly what gear oil and what additive they used and how much and write that down it your owners manual for future reference.

 

bunch of posts on the forum recommending 75w90 dino oil w/ a tube of motorcraft LSD additive squeezed in

not doing all that. Picked syn LSD oil that had the additive already in it. First probably Motul, then later went BG 75w140.
No issues.
Except for the fact it's a clutch type LSD designed for a stock truck on 31" or smaller tires and likely does not function with common bigger tires

whatever works