Sture and Dave's advice is spot on. The failed part is likely to be the large o-ring. When you mount the "new" final drive, leave the three driveshaft tube bolts less than tight until everything else is tightened, then loosen them a bit and torque them up last to prevent "building in" any unwanted mis-alignment stress. Only fill to the bottom of the side (fill) plug hole, don't fill thru the top vent hole. Honda moly grease (special and costly) or it's equivilant should be used on the wheel and hub drive spline set. The moly content wants to be very high, more than 50%, equivilants are rare, many greases say "moly fortified", but 4-6% is common and inadequate. Do this whenever you change the rear tire. The previous owner probably did not . . .