Here is a lead for you, military trucks of the era.
Have a look at this Ebay listing:
Rear Inner Axle Seals ...and this Ebay listing:
Rear Outer Axle Seals I "think" those parts are the same as yours, not sure because of the "split axle" designation. Click on the seller's "see other items" to discover what all he has. I would email him and ask about parts for your truck, civilian, military, the same.
His listing is for a "CCKW" which is a 2-1/2 ton GMC. Don't know the military designator for yours. However, many parts, 1-1/2 ton through 3 ton are the same. Look around the web, you will come across whatever military symbol covers your truck.
Point here is there are a lot more military truck part suppliers than ordinary part sources we often use. You will have better luck through those. An example is I found old Clark transmission parts through a military parts source on the web and on Ebay.
I will pull more info and get back to you.
Edited in note here, your truck during WWII is designated "ACK-353" and the Chevy version is designated "G506". Those would be good search terms for Google.
Billyray
