Have England accept the Jews when expelled from Spain in 1492?
Of course, IOTL, England had illegalized Jews at the time, so it's not likely, but it's the most plausible way I can think of.
Jewish majority is completely unviable.
That would be interesting.Otherwise, you could have a ridiculous Khazar-wank or similar result in Jewish Vikings.
The way to do it would be to have the Slavs converted somehow (possible, if unlikely), before the Rus arrive, then have them spread Judaism further into Scandinavia.That would be interesting.
Makes me think of "The Crimson Permanent Assurance".