It sounds to me like the differential bearing carrier seal? There is also a seal on the final drive (behind the brake). The old ones were leather.
It may be wise to check the level in the transmission, but if you are going to do that may as well drain it and change it. (HyTran recommended). They are know to fill up with water and the easiest place to drain is out the differential seals and then onto the brakes and eventually out the drain holes under the brakes.
1971 Cub (Rufus) 1950 Cub (Cathy) 1965 Lo Boy Fast Hitch (Nameless III) 1970 Cub 1000 Loader & Fast Hitch (Lee)