Have a look at the shop manual for the two speed axle in question, a double reduction unit as mentioned and explained by Kenn.
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0428.htm

The ring and pinion come first, then it uses either of two final reduction gear sets. Nice picture in that link.
No planetary gears (as in later 2 speeds), and always double reduction.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-