Here's a place you could start.
http://74.125.47.132/search?q=cache:MgkqjRZXjCQJ:www.ratechmfg.com/gmaxlebear.htm+1963+gmc+axle+repair+bearing&cd=1&hl=en&ct=clnk&gl=us
I would recommend that you go to your best NAPA store and hang out with them for a while and see what they come up with. Timken, BT and others make them. Is this a stock 1964 Chevy truck rear end, or what? The truck 12 bolt was first used in 1965. Before that it was something like an HO52 OR SOMETHING LIKE THAT! Don't quote me on that. If you know the rear end, then you'll know what you need to hunt down. Best of luck.
