You'll have to drop the driveshaft, and drain the Trans, and/or just put a catch pan under either way.Its called the rear extension housing seal. Pry the old one out with a seal puller or screwdriver. Replace the seal square to the extension housing bore with a socket which should be slightly smaller than the outside diameter of the seal, tap with rubber mallet til flush. Reassemble and check fluid level and done.
Theres your part#, same on all models for that year from Ford, and not much more than what you were looking at on O'Reilly. Personally I would go for the OEM part, that and I don't care much for O'Reilly much around my neck of the woods. Good luck!
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