Don't panic. Unless you see a puddle directly under the differential, your leak may not be as bad as you think. Despite any leak - perhaps the most important concern is that the leaking component - remains lubricated rather than "dry."
Locating a leak source is fairly intuitive.
Differential lubricant typically contains a high pressure additive that has a distinctive “sulphur” smell. In other words, the smell may indicate differential lube - as distinct from engine oil or transmission lubricant. When was the last time you checked the differential lubricant level? Could the differential have been overfilled? If it is low, or any other potential source (engine/transmission) is low, this may suggest a leaking component.
sfDon offered a likely diagnosis, but if the nature of the leaking lubricant is not readily apparent, per his suggestion, you might consider cleaning all areas covered with oil/grease/dirt and inspect for “fresh” oil, followed by checking engine, transmission and differential oil levels. Consider the effects of gravity and forward motion "wind" - and the tendency of forward leaks to accumulate at or near the differential.
Once you determine any leak source/s that should indicate remedial efforts, e.g., snug up related fasteners, replace failed or failing gaskets and top up related lubricant.
Good luck.