The rear housing should have a bronze type bushing pressed in that the input yoke actually rides on. If the bushing is worn or the yoke has a groove or substantial wear, a seal won't fix the leak. It will be necessary to remove the rear housing to replace the bushing.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine