Nissan Patrol 4x4 (Africa)

Is it normal for the cruise control of the 4.8 Auto to disengage when the selector is moved to the manual position, in other words, I cannot use the cruise control when driving in manual mode?

Yes its programed that way. Cruise control will automatically disable when you switch to manual. There is a workaround to enable this feature. I remember reading in Australian Partol forum.