Maybe it's not the trans seal at all. I have seen them leak at the splines (trans output shaft to diveshaft flange). The cure is #2 Aviation Permatex (in a can with a brush) on the washer, under the nut for the driveshaft flange. If you pull the nut and washer off, you will see it wet there. GM made a washer that was coated with a rubber like substance to be used on some trans. Take the nut off and look.

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