Thad,
I'd guess most of them DON'T have drain plugs. I just checked all the rear ends in my shop and none of them have drain plugs (including my '38 1-1/2 ton). On rear ends with the rear cover, it's just a matter of pulling the cover off and letting it drain.
On the Corvette (same as 55-64 passenger), there isn't a rear cover so you either pull the axles and loosen the center section or NAPA sells a suction gun that you can use to remove most of the old lube. The gun also works for putting lube IN if the fill hole isn't accessible otherwise.
