Thanks for all the info so far. If anyone has anything to add, you can be sure I'll relish the info.
I do believe my truck came from the factory with the two-speed rear axle. It is a 1959 model, sorry I didn't mention that outright from the beginning, it might have saved some confusion. According to the registration papers that came with the truck, that date every year back to around 1965, the truck was once a tanker for a fuel company out of Durango, Colorado (It was converted to a flatbed sometime in the 1980'). So hauling that kind of weight, it probably had the two-speed. (It's original GVW was 19,500, thankfully the New Mexico MVD let me "de-rate" it to 10,000 lbs, which saved me a lot of moolah on the registration fee!)
It does have the mechanically operated vacuum switch on the left (driver's) side of the tranny. I'll have to follow the vacuum lines and see where they go. The rear axle hasn't shifted since I got the truck a couple months ago, and it could be that the line that ran to the speedo adapter is just hanging there, leaking vacuum, which could account for the non-operation of the rear axle.
There definately is no adapter on the back of the speedo, and the speedo is not working anyways, so I'll be on the hunt for a whole unit, complete with speedo adapter.
