I don't see a plausible way of doing this.
If Japan were to be a US ally in this scenario, it would operate under the US-German aegis, or just the US's if by some miracle Germany was defeated.
Basically, it would require Germany to be quickly trounced in the war before the US got involved; I'm going to assume this is due to the USSR, France, Italy, and the UK, all allied to one another (maybe they've all gone communist; that's probably the only thing that would ally the US with Nazi Germany in the first place), launching a surprise attack on Germany. I never thought I'd say this, but poor Hitler.
Anyways, then it would require the People's United Republics of Britain or whatever to heavily militarize Canada. This would be a really nasty war, because the US-Canadian border is quite possibly the single most indefensible thing in the world. The US would be able to easily capture the major cities, but the rest of Canada is going to be a total quagmire not unlike invading Russia. Also, if the Comintern gets naval superiority in the Atlantic (not sure how plausible that would be, probably not very) it would be nigh-impossible for the US to keep Britain from resupplying Canadian and Caribbean airbases to stop bombing raids and so on.
I'm going to further assume that Mexico has somehow gone communist and has both industrialized and militarized. If the US is somehow forced into a two-front war (with one of them being an occupation of Communist Canada), that might be enough to grind public morale down and force an armistice. Then the US economy would be ailing enough so that it doesn't develop the A-bomb until after the Russians do. Let's say that the US tries to go completely fascist and collapses into a civil war in the 1970s, with nukes being used and large swathes of the country being reduced to rubble. Japan negotiates an alliance with a Pacific coast successor state (California, Oregon, Washington, and perhaps others) to be included into its co-prosperity sphere. Nationalist China, which has more or less defeated the CCP, also joins in.
There is now a nuclear Pacific Bloc with Japan at the forefront facing down a Soviet Eurasia. Almost 1984, except Oceania has fractured into mostly neutral successor states and Mexico is probably still communist.
Pretty ferociously unlikely overall. I'm not even going to try to find a POD that would satisfy the above conditions while keeping the rise of the NSDAP intact.