Ford Gen. 1 and 2 6F35 transmissions are notorious for worn axle bushings, which allow external leakage. This premature bushing wear can be caused by a worn differential support, which affects endplay negatively. The thrust bearing that rides on the differential support rotates, wearing deeply into the aluminum support and allowing endplay to become far excessive. To correct endplay, a new case half is required.
Instead of sourcing an expensive new case, you can stop further wear and restore endplay to proper levels by installing Sonnax case saver kit 144281-01K.