Loss of drive to rear wheels

Defender 200tdi 110. Ok, have been getting a rattle when going downhill in gear but backing off. Though it was propshaft UJ's but not long since I did those. Jacked her up today. If I jack both rear wheels free of the ground and put it in gear, it tries to pull away from the front ones, with no drive to rear. If I jack front wheels free of the ground they spin and still no drive to the rear. Rear propshaft doesn't spin but does have a touch of force there. So, any help on how I diagnose what this is please? Gear box or transfer box? Anything I can take off to see whats happening?

Jacking up the rear and then trying to drive away at the front sounds an interesting thing to try.......rather you than me. However, the centre diff should send all the power to the wheels with least resistance, so with the rear jacked up the front wheels should not have had any drive. There would be power to the front only if the centre diff were locked.
Is the front diff locked on the lever?
Did the rear propshaft turn?
If you jack up the front and try to drive the centre diff would send all power to the front wheels anyway so that experiment proved nothing.