Pinion nut backed off of the pinion gear of the differential. The nut is captive within the coupler. Remove the six bolts holding the driveshaft joint (coupler) to the pinion cup. The nut and washer will be in there. There is the slimmest chance that you could simply reassemble (with a new pinion seal) and carry on.
There is a somewhat higher chance that the pinion, before the nut came all the way off, may have caused some damage to the gearset due to its being unloaded and a bit misaligned.
Either way, some noise was made and some noise was ignored.