When you say "every now and then", the diff can slip under high load, that's what it's there for. It makes a "barking" kind of noise if it's set properly, and sounds AWEFUL, but it's doing no harm.
THey key is, the slipper should be whining whenever you give it full tits from a stop or on high-bite surfaces, the diff should only "bark" when you swing into a tight turn at speed.