A 2 ton AD (Advanced Design) generally, but not always, came with 2 speeds. It's the 1.5 tons that were standard with a single speed rear but a two speed was an option.

This is the vacuum valve/switch for a two speed:
http://rides.webshots.com/photo/2662657990080251109jBhLTF

I don't have a picture of a two speed rear handy, just checked.

Grigg

•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-