I suspect you blew the pinion seal or fluid was being blown out through the vent.
The reason, probably overheating of the axle.
The carrier and or pinion bearings were either getting bad and caused it to overheat or the axle was low on fluid causing it to overheat.
The repair, replace the rear seals, bearings, possibly gears, and possibly the brake shoes if the axle shaft seals leaked..
An alternative and possibly cheaper solution would be to find a wrecked low mileage truck or Suburban and swap in the whole axle.
The reason, probably overheating of the axle.
The carrier and or pinion bearings were either getting bad and caused it to overheat or the axle was low on fluid causing it to overheat.
The repair, replace the rear seals, bearings, possibly gears, and possibly the brake shoes if the axle shaft seals leaked..
An alternative and possibly cheaper solution would be to find a wrecked low mileage truck or Suburban and swap in the whole axle.