treat the vacuum lines like water. if the result is not correct at the end point (cad), then either start at the source and follow it forward step by step, or start and the end (cad) and follow it backwards.
prove you have vac at the engine.
prove you have vac at the t-case.
remove the vac switch from the t-case and test it. note the 4 ports and the keyed connector. it can only go one way. one line is input, one is vent. one is 2wd output (inside button not depressed). other is 4wd (button pressed). bench test it. reinstall switch and move to the cad.
as said the steel vac lines along the transmission cross member get torn up. you can replace with anything. brake line, fuel line, copper line, aluminum, rubber hose.