Is there a way to make diddles sound even in Finale 2008?...all my playbacks make them sound really overlapped.

I would personally use the TG TOOLS ";SMART PLAYBACK"; plugin.  Th is can be found in PLUGINS -->TG TOOLS -->SMART PLAYBACK.  You will want to highlight everything with your SELECTION TOOL first and then process the document with this playback.  You can even do this to specific measures.  Maybe you only want to do it with the measures that have rolls....this will do it for you.