Bridges and underpasses etc

[Steve]

If you plan to go wading in your 4x4 make sure the diffs are properly vented, ie the vents are clear and connected to pipes that go above the waterline. Diffs can get hot in poor ground conditions, and suddenly cooling them in water makes the air inside contract which can suck muddy water inside, or even displace the gasket if the vent isn't clear.