I have a '67 c50 with a vaccum shifted 2 speed axle. The 'switch' on the shifter is attached to a cable that goes down to a vacuum switch under the floor. My truck had sat for years and it was stuck. I pulled the cable and lubed it and put it back together and it works. I don't know any thing else about it but if yours is the same setup I would check the vacuum lines from the source at the engine back through the switch and to the axle itself.

Good luck and let us know how it works out.
-Richard

40 chevy 1.5 ton, 50 chevy 3/4 ton, 67 chevy stepside 1/2 ton, 67 C50, 68 C50, 73 chevy 4x4 3/4 ton, 2004 Silverado 1/2 ton , 2005 HD 3/4 4x4 Duramax (don't even get me started about my Oldsmobiles!)