gearbox or transferbox

Hi all
Been out today and when driving i get not a load noise but a noise and its like a vibration and more so when it gos in to overdrive , is this the transfer box or the auto box
sorry this is a 300tdies auto
if its the box how hard is it to change,if the transfer box can i just change that all dos all off it have to come out

sorry but if i need a new box will all discovery autobox's fit what about the transferbox are they the same as a manule or do you have to use an auto one

sorry but this is my fist discovery and i think i was sold a lemon