Mark and Mike,
Thank you for the replies.
I am guessing that the current vacuum gauge was put in to replace a gauge that came with the truck, but it is not hooked up.
I had not thought about an adjustment for hi- and low-range speedometer readings - perhaps because the truck was stuck in low range when I bought it, hopefully because the button cable was frozen (and now works after much treatment with penetrating oil). The vacuum line I am asking about definitely comes out of the hi/low switch, but could still be headed to the speedometer. I will start by capping it off so that I know it doesn't interfere with tuning the engine, then connect my vacuum gauge to it so that I can tell when, and to what degree, it is active.
