The seal is available separate ( I dont have the P/N handy). Changing is relatively easy, too
To remove the output flange you need a drift to stick in one of the bolt holes (place it in so it pushes against the case webbing and keeps the flange from turning). There is one 19mm bolt that holds it in and is not torqued down very hard (about 25 ft/lb, I dont know off hand and dont have the manual with me). After the bolt is removed the flange pulls right out. You can get a seal puller from Harbor Freight for about $5. It looks like a rod with a two headed hook at the end. Pull the seal and push the new one in until it is flush with the shoulder within the bore.