In my experience these usually leak when a diff has been made up out of bits and bobs from odd diffs and the oil creeps down the splines then under the large washer and nut, And then drips from this point and gets slung around under the car. A semi flexible sealant such as a Wurth product can be used but if the splines are actually well worn then you will probably find that oil still creeps through, Well worth trying to get a few used or a new input flange to try. But not the easiest job to keep changing them chasing an oil leak