Looking for advice on where to start, what started it all was a light growl from the transmission and I noticed the 4wd wasn’t engaging. After multiple attempts it when into 4wd noise went away. Put it into reverse and lost 4WD again, no noise. selector switch appears to be working correctly but I am not sure to where to begin looking into why the 4wd isnt always working.

Can someone brake down how the 4wd unit is suppose to work on this, i know the front output shaft always turns but is it really engaged on the transmission side all the time and front diff just engages or is it simply spinning from centrifugal force?