You're welcome, and please let us know about the follow up with the Audubon society - they should appreciate your info. I'll bet what you were seeing was a foot and leg. Grebe's legs are set far back on their bodies. I've seen them do something like this before. I remember how weird it looked to me at the time before I finally figured it out.
I think paying attention to range maps is to be commended. For one thing, it tells you when you need to be extra careful, as extraordinary claims require extraordinary evidence. I really like what David Sibley does with his range maps, showing areas where the bird is rare. I can help people to tell the difference between what may be a rare sighting and one that is perhaps unprecedented. His online guide is here: http://sibley.enature.com/home.asp, but I would highly recommend the book version, too.
It sounds to me like your evidence is very strong for Western Grebe. Congrats on a nice find!
Randy