Backlinks and Content is what matters for all engines. But you can buy your way through Yahoo. Use Yahoo Search Submit. There are various options. I have gone in for the Basic program. It is very good. +rep appreciated.
Use site maps and submit sitemaps. Sitemaps in HTML and XML format. Then use Yahoo related services : Yahoo Answers with your link in the answer boxes. Yahoo webmaster pages. Tagging and Pinging with various services : Technorati. Feedburner. etc. etc. Bookmarking websites : Digg and Stumble Upon. netscape and Propeller. Basically yahoo related services carry a lot of weight, the same as Google related services carry a lot of weight in Google.
I think yahoo's algorithm favors the quantity of backlinks you build rather than the quality. If you build enough backlinks your ranking will improve in yahoo's serp.
I'd have to say with my experience with Yahoo and google you are 100% correct If I show up in Google for a key word, it NEVER shows in Yahoo, if I show in Yahoo I can be sure it won't even register in the top 100 for google. the only conclusion I can come from this is it's no fluke. yahoo monitors what key words google brings you up for and will not list for the same maybe not in every case, but it's pretty close