I plan on dropping my FR-S in the future, and I'm definitely interested in this topic. On my previous car (Altima Coupe), I had axle problems as well once it got low enough on coils. After going through a few sets of axles... I gave my axles to this guy
http://www.gatorracingaxles.com and they held up really good, and all he did was upgrade the boots and grease.