They are super easy to replace so call around and find out the cheapest place you can get them. This is something I'd do for $20 if you were local to MN but I'm guessing you're not so see if you have a friend that is handy with a wrench and save yourself some money if it's convenient for you to do so. It's a 5 minute job for the dealership once it's up on the hoist and the sensors are about $100 a piece depending on where you get them. If you want to call around and price shop for the best deal, the part numbers you need are:

BOSCH Part # 16123 - Driver side downstream O2 Sensor - $92.79
BOSCH Part # 16276 - Passenger side downstream O2 sensor - $129.79

Just do a search on www.rockauto.com for those part numbers if you want to order them online.