Are you talking about the rear axle or the rear crankshaft seal?
I believe those engines did not use a rear main crankshaft seal, rather a slinger. There are various issues that could be occuring, look for "slinger".
IF you are talking about the rear axle, there are seal kits available from the major Packard vendors, Max Merrit, use their online search tool for 1940-42 and search for "axle"
Max Merrit And Kanter
Kanter 1940-50 Packard Parts There are a few bearings which have been added to the cross-reference on this website:
Packard Parts Cross-Reference, Rear Axle Components, this websiteThis Post was from: https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=256714