If it's a ball diff, they typically make a "barking" sound if they slip. A common method to check, pull the gear cover and motor pinion gear off, either hold both wheels or put wrenches in the outdrive slots to stop them from turning, and turn the spur gear by hand. The slipper should slip, not the diff. The top shaft & nut should not be turning. This is something that should be checked regularly, as the diff wears a little and the spring takes a set over time.

If it has slipped, check for smoothness by turning wheels in opposite directions. It's likely a rebuild in needed, new balls and new or flipped rings.