Pinion yoke cap loose?

Looks like a 489 third member. Those are dust covers and are best to pull the yoke and get rid of it rather than worry about it coming loose again. Your is a crush sleeve set up. Before you remove the nut , grab the yoke and see if it has up and down or in and out play. This may be the cause of your leak. Also need to Ck the yoke seal surface to see if it is smooth and not groove. FSM will tell you when you remove the yoke, you need to replace the crush sleeve and set pinion preload.