The seal can be replaced in the car. The driveshaft will have to be removed. The seal is a common part available in most parts stores. That may not fix the problem, however. If the rear bushing is worn the seal likely won't last very long. The rear tail housing of the transmission will need to be removed 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