I've heard of a trick where the VW guys weld the spider housing shut, with a bung added, and fill it with gear oil, then run it till it drops. You could do sorta' the same thing with old Japan axle, but just run some grease in the spider gears and shaft. There won't be a lot of diff action unless a tire is spinning a lot, and the grease will be thrown against the inside of the housing. I don't think you'd need to worry about sealing the axle shafts into the side gears.
JimL