The end of March / beginning of April is usually when our rainy season starts. Most thing should be running again, although must our runs are dictated by the last day or two of rain so to plan months in advance isn't really possible.
You should check out the Chasing Jaguars
whitewater guide book by Lee Eudy, it's available at Amazon right now for pretty cheap, about $16 or $17 I think.
Also we have a web forum, Costa Rica Creeking - Whitewater Forum
that might be useful to hook up with other paddlers if you want to share a shuttle.
Also I sent you a pm in case you want any more info from me or our guides at Pacuare River Tours.