I would think you should be able to get seals and gaskets for that axle, it's not ancient and Eaton is still a major supplier for trucks.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy