Rear main seals are kind of an age old problem on Willys vehicles. I have experienced the frustration of working on them in the military and later on my own vehicles and in shops. The last one I replaced on an M38 was a neoprene seal and it has lasted for several years. I have also found that parts offered by some of the military suppliers are NOS and could be 50 years old. In some cases parts have been so hard or brittle they don't work very well. Midwest Military and Ron Fitzpatrick are good sources.
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