Andy will be along soon but this is what I have gathered so far.
car felt good for approx. 15 miles or so then after a decent speed run up the motorway and checking all the gears selected ok in manual/sport mode we came off the motorway and went through some twisty sections and roundabouts.
During the last turn Left to right, the car started to judder as though the wheel speeds were trying to be controlled unsuccessfully. Then as we gained speed up an inclined slip road the rumbling noise was definitely there, the rumble noise peaks and troughs, as in resonates, its not a constant sound. after that the noise came back as it had done before under light load low speed incline.
*** EDIT we did experience the noise at 70mph too ***
My instinct is saying differential as it sounds like its coming from the back end.
But, we are not completely sure that the transfer box is functioning properly because it doesn't actually work in the high/low range change. Fuses are in place but the switch doesn't trigger the actuator.
Think Andy is going to go for new tyres which are needed. The rear tyres are unevenly worn and near limit on the front, so good chance that the circumferences are wrong. The wheels are 20 inch if this makes any difference
Looks like the front and rear diff oil should be checked and renewed and then possibly looking into the service of the CV joints around the whole car as we have no idea if they are ok or not.
Gear box sounds good. it all functions very smoothly. Leaks are now gone so hopefully we can try and figure this rumble having the Gb and TC taken away from the equation.
I need to check but are there wheel speed sensors on each corner that send data to control power distribution?