How do you change the rear output shaft seal

Don't know if you've already figured this out, but...

Mark how the driveshaft lines up with the yoke and drop the drive shaft by disconnecting the front U-joint, remove the yoke (it just slides off the end of the shaft). You want to make sure it goes back the same way or it can cause U-joint failure. Also if the shaft of the yoke is scored or worn it needs to be replaced or it'll leak again. That's a very common problem.

Pry the seal out. You can get a tool that looks like a a small pickaxe that makes this easier, but I always just use an old flat screwdriver that I sharpened the blade on. Being extra careful not to scratch the transmission casing where the seal sits use a hammer to tap the sharpened screwdriver into the seal and pry it out.

Wipe any oil around the sealing area off. Get a deep socket or similar that is a bit smaller diameter than the seal. Place the seal up against the transmission tailpiece and align the socket. Gently tap the socket with a mallet or hammer until the seal is driven flush. Replace the yoke (oiling the tip of the yoke can help it slide in) and reconnect the driveshaft.