Kent,
I only know about the 2-speed mechanism in my 54/55 GMC 2-ton truck.
It defaults to low speed when there is no vacuum. I was told this was logical, in case you lost vacuum with a heavy load - you could still get a heavily loaded truck rolling?
I do not see a clear statement of the "default" speed (low or high ratio) for 48-53 Chevrolet trucks, but the Chevrolet 1948-1951 Shop Manual (which applies to 52/53) seems to indicate in the diagnostics that loss of vacuum results in low gear/range? (
bottom left of this "Troubles and Remedies" diagnostics page )
