I'm not sure if there is a cleaner solution, seems like there ought to be something available from a later model medium duty truck with 2 speed rear...

But this may work with a switch to put it in or out of the circuit as you shift the rear axle.
http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=126/


•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-