It's pretty easy - pull the rear driveshaft, prise out/pull out with pliers the rear seal, and replace. The trick is driving it in evenly - I wound up drilling out a hole to "miss" the seal boot with a hole saw in a chunk of 2x4, and I lay that on the seal rim and drive it in with a hammer. This allows me to clear the output stub and bang on the seal fairly evenly, without damaging the thing in the first place.
Expect it to take an hour or so.
5-90