Ahhh- there's a catch.
Since you mention you have HDC and TC I'm assuming you have one of the majority of Series II Discoveries which weren't fitted with a Central Diff Lock.
Land Rover fitted TC to Series II Discoveries and thought this would be so wonderful that Diff Lock was deleted. The old Diff Lock only worked on the two axles, not each wheel, you could still get stuck.
TC in theory works better than this, but it requires the wheels to spin before the system kicks in. On mud this means that the wheels may churn themselves into the ground to the extent that TC won't get you out.
From 2002 the Disco had the Diff Lock back as an option. It was found that a combination of good old mechanical locks and high-tech electronics worked very well.
If you have a SII Disco with Diff Lock, it's fairly obvious. The transfer (High/Low) lever will have a second position marked on it- 'Diff Lock' surprise surprise. Push the lever over to left (no clutch needed and you can do it on the move) to engage diff lock, simply flick it back to disengage.
You can get kits to re-fit the Diff Lock to SIIs without it.