I'm learning Gershwin's Blue Lullaby (2nd prelude). There's a section where you play with hands crossed, melody in the lower notes, and that section causes sharp pain in my right shoulder. I have a mild case of arthiritis. Anybody else experience this? What can be done about it?

If it is serious pain, then see a doctor. Always better to find out what's going on to make sure it isn't something serious.

You could try angling your body a bit during this part, turning your whole torso to the left a bit. This might facilitate the crossover a little bit.

Or you could play it without crossovers at all. I don't play this crossed, I take the melody with the LH. If I'm mistaken, I think the score marks it as an optional crossover, so don't feel like you have to do it that way. I've tried it both ways and find it easier to just play it with the LH, but to each their own.