A friend here used "Mean Green" (a strong degreaser; available at WalMart) which took quite a bit of the yellow off. I think other similar cleaners (KrudKutter etc) would have similar effects. With more rubbing I noted it even began cutting through the base paint layer. Its not a precisely controllable process but I think if you worked large areas while paying very close attention to the resulting look it might work out ok. Try it out on the cab back first and see how it does...
edit: when looking at what came off on the cleaning rag, it seemed like much of the "yellowing" was likely nicotene stains. It was more brown than yellow.