How to convert RMII signals to LVPECL signals
Curious about the application, but as regards the actual conversion question I've had great luck with On Semi's range of stuff for clock distribution. 

I use it for generic differential to single ended stuff all the time, it's not just for clocks. They've probably got a signal converter for ya:

http://www.onsemi.com/PowerSolutions/parametrics.do?id=648