try this before changing the gearbox oil. Replace the gear oil with new oil.
Run a 75/25 of manual gear oil and auto gear oil. Usually on small cars they do a 50/50 but i think that might be too much for a truck.
It may sound stupid but i've seen it fix some crunching boxes. Its also cheaper than buying a new box.
if that doesn't work try more gearbox addative to stop crunch Moories somthing i can't rem but supercheap stocks it.
Give that a go before you start taking the box apart. It might work as my friend did that first in his Hilux when he had crunching problems. His went away.
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