my mate has bought a navara 4x4 ute and has found that when he changes down gears it grinds like the sincroes are stuffed but when he changes up gears its all normal and quiet. i dont know the year or model sorry. the truck has done deep water crossings in the past and the bottom of the clutch master cylinder fluid resivior has about half an inch of silt or dirt in the filling point, but remember it changes up gears all good?
What does the clutch feel like? If it was me i would be changing that clutch fluid and bleeding it before getting all carried away and pulling the box of etc, Do the simple things first, if you do it right you can change up gears without the clutch without any 'graunch' but down changing is alot harder so it still could be the fluid, could be the clutch its self or the box. But yea if it was me i would start simple before pulling it apart
Never argue with an idiot, they drag you down to their level then beat you with experiance!