Re: Manual or automatic gearbox

Overdrive & torque converter lockup are two different things.

If you want to check for torque converter lockup find a clear straight road with minimal traffic, you need to be able to get up to around 60 km/h - make sure the OD disable button is not depressed, (the OD OFF light must not be on) - accelerate in Drive gently from a standstill, keep an eye on the tachometer, and count the shifts, 1>2, 2>3, 3>OD(4), watch closely after it shifts into OD, within about ten seconds or so, you'll see the tach drop a few hundred rpm, that's the torque converter locking up.

Now, with your left foot, very gently depress the brake pedal, just enough to bring the brake lights on, but not enough to actually apply the brakes - you should be able to see the revs increase when the torque converter clutch disengages (when the pedal is depressed), and decrease when it engages (when the pedal is released).

Now repeat the exercise with the OD disable button depressed (OD OFF light on), this time you should only see two "up shifts", 1>2 & 2>3 and the torque converter should lock in third gear.

Does the auto box lock the torque converter in low range at all to enable better engine braking? I know some vehicles like a Disco 2 or a p38a Range Rover have this ability.

The auto is a 4 speed with an overdriven 4th gear. Not a 3 speed with a separate overdrive.  In that respect it's similar to the earlier 5 speed manual pre 2005 which also has an overdriven top gear. 
The control computer will only shift into 4th above 73deg Celsius and only locks the converter once in a steady speed and load condition. 

The box can properly lock the converter in low range but the engine displacement limits its ability to brake just like the manual it's not a 3.0 diesel after all.  It will also change up gears if the selector is left in D. Usually though a modest decent is manageable in 3 or D with just covering the brake it's only the most extreme and or prolonged decents that need locking in L.