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JAL and UA operate from different terminals at NRT so you will need to either clear customs and immigration or else transfer airside between terminals.

http://www.narita-airport.or.jp/airp...ess/index.html

As long as JAL and UA have an interline agreement you should be able to check bags all the way through from departure point (just tell check-in agent want bags checked right through). They do this sort of thing all the time.