Locking CDL and keeping roof loads down
If anyone is still in doubt about locking the CDL as often as possible (ie whenever off tarmac), watch this video. Land Rover Defender Talk. Andrew St Pierre and John RIch - YouTube
Also, IMO, it might help to think about the Defender 4x4 system a little differently. Instead of seeing it as an on-road system that can be occasionally locked up for off-road Think about it the other way round. ...Defenders are primarily off road vehicles. As such, the CDL is designed to be locked all the time. Unlocking it simply enables you to drive on bitumen without causing transmission wind up. So leave the CDL locked, unless you need to drive on the tarmac/bitumen. Simple.
Roof loads? It’s an absolute no brainer to keep weight low. A roof rack is for a wet swag, perhaps a second spare tyre if on an extended overland trip and collecting firewood close to camp. Keep all other heavy objects such as tools, fuel, gas bottles, water, etc off the roof. Pack lighter. It’s too dangerous to be top heavy.