sorry, but your friend is wrong, there are no vacuum lines for the 4WD system inside the cab of your truck.
it'll be for your enviroment controls, likely the vacuum diaphram that is located to the right & above your gas pedal.
as for your 4WD not working properly....will it go into 4LO without any issues? if so, then it's your TCCM (Transfer Case Control Module) located behind your passenger kick panel. if you have ever had any water leaks over there (actually quite common for these S-series trucks) then it could be shorting out, or simply just worn out. quite common for these trucks as well (i keep 2 spares on hand myself).