diff breathers

I don't know anything about the X trail, but... if going through water crossings diff breathers are essential. Diffs change temprature depending on circumstances, hence the need for breathers. You don't want the diff sucking moisture or sand at the wrong time. They are not supposed to let water ecetera back in, but it happens. Anytime you get air moving in or out it's going to happen.

Either fit breathers or check your oil for water as soon as possible afterwards.

YES is the simple answer

For a Cape York trip diff breathers are essential and more so than a snorkel. People do not understand that most diff breathers, particularly those fitted to SUV but also a lot of 4WD sit on the top of the axle. So even crossing water at hub height will casue it to flow over the axle and allow the breather to suck it in with subsequent damage to diff and axles. There are plenty of reasonably priced kits just make sure the breathers are placed in the engine bay on the firewall as high as practicable

Regards

RobA

The T30 Xtrail already has diff breather tubes - the front has a hose going up near the airbox and the rear has a hose going up into a crossmember.
Link here

I'd be more worried about the air intake. You'll need a blind for any reasonably deep crossing.