If it is a big bearing rearend pull the axle first to make sure it doesn't have tapered bearings. If it does then it takes a different seal and bearing that is sealed outside the bearing (The bearing bathes in the rearend gear oil instead of being sealed with grease). You can always change it but keep in mind with a sealed bearing the seal is in the housing itself, with tapered it goes on the axle before the bearing. It is important when changing types to either add or remove the seal in the housing depending upon the type used.
