that would be my guess as well... obdI ludes only have the 1 o2 sensor before the cat so it would seem that the secondary one for obdII would be after the cat.
you might want to opt for a generic bosch o2 sensor if you dont' want to spend the 200+ for an oem one. I picked one up locally for around 70 bucks and it works fine. you just need to splice the wires and reuse the connector from your old sensor. also, i think
www.oxygensensors.com might sell one with an oem connector too. some people will say that the generic one's don't work as well but I really can't tell the difference.