GU Patrol 4.2td Front Hub Problem in 4WD

Take it back.

Are they auto locking hubs or manual ones?
There is obviously a sticky mechanism inside one hub.
Maybe need to be pulled apart, cleaned and greased.

Can't think how the nissan locking hubs work without looking at one again, that said Toyota's hubs are known for playing up when the wheel bearings are getting tired. So have you checked the bearings??

I also have a Nissan GU Ute and found up at Cape York that the front manual hubs unlocked themselves whilst driving along. They unlocked in both sand driving and on dirt roads, spoke to Nissan, they acknowledged the problem but offered no fix!

first thing that strikes me is why are you driving on sand with tyres fully inflated.
try doing it with the tyres at proper presure for sand(usually less than 20 psi) and see if the problem persisits.
cheers

Does the front wheel just loose traction? If the left wheel looses traction, all power to the front is transferred to this wheel. This possibly could be attributed to tyre pressures. But this doesn't explain the clunk noise.

Leroy

Auto hubs are a common fault if they are playing up replace them with AVM hubs for about $200.00

Mangopickle,
You say the hubs are locked in correctly, is the arrow in the auto setting or the lock position? If the hub is in the auto position and you are switching between forward and reverse there will be a certain amount of free movement between backward and forward motion, if there is power on this will be a nasty clunk and quite a shudder. If you are driving in sand it's better to have the hubs in lock, if you are aware of all this sorry. Accelerating and decellerating in auto lock can be quite frightening especially on gravel and up and down hills. My opinions and experiences etc etc