Gear differentials don't slip unless there is a mechanical problem such as broken teeth on the gears.

Losing fluid, or thinning fluid will cause the unloaded wheel, or the front, or rear to "diff out" or spin easier. It's like when the clutches wear out on a posi-trac axle on a full scale car, it starts to behave more like an open differential instead of driving the wheel that has lost traction.

Jay