diff breather for sure. You should also have one on the front axle, and also gearbox, transfer box and fuel tank (maybe)
When the truck is driving along the oil in the diff gets hot and expands a bit, this is normal. If the diff and axles were sealed air tight (as you may expect in order to keep the oil in and the **** out) then when the oil expanded with the heat it would force its way out of the half shaft or diff oil seals and knacker them, so the breather allows a pressure release.
Also it has a long tube on it because as mentioned the diff gets hot when driving, if you then forded a stream or river the diff would immediately be cooled as it was immersed in water. This creates a negative pressure resulting in air being drawn into the diff. If there was no pipe to extend the breather water would be drawn into the diff, which would knacker it over time.
If you want to extend them most extend them into the engine bay.
The ones on my Disco are taken to the top of the snorkel, and this is front diff, rear diff, fuel tank breather, transfer box and transmission breathers.
