Failing 4wd on my Dmax changed the change over controller on the back of the transfer casing and the actuator on front axle . It selects 4wd but I have a very loud grinding then it engages after grinding and crunching . After closer inspection to the oil in the transfer box it's black with a hint of glitter . . My guess at this time that the transfer box is buggered. .and when driving under strain ie not changing down quick enough I have a loud rumble sound like a failing bearing . .

 

On my Ex's 2013 MuX.. Her Old Man Smashed out the Housing on the Front Diff. looked like the Input Shaft Bearing exploded..
Still dont know why it happened but maybe thats an area worth having a look at?