It's a 3/8" allen(hex) socket. You can't change the fluid enough on these rear ends. The fluid gets extremely hot and sludge forms within a few thousand miles. If it's been in there for quite some time I would pull the rear and clean it out. Perfect time to drill and tap the housing for a plug. Note DO NOT DO THIS WITHOUT THE FACTORY SERVICE MANUAL! There are critical installation points to observe, including new straps and bolts.