Must be too busy to check the two sites I mentioned and linked to above...
Your exhaust pipe should look something like this:
The sensor you need is the same regardless of position. AC Delco AFS105 or GM part number 25312200. RockAuto.com
cost of $58.79. GMPD.com
cost of $76.58 with a GM list of $147.26. Parts4chevys.com
cost of $92.04. Go through the process and see what the final cost and delivery time would be from RockAuto.com. Use that as the price you want the dealer to hit. If they don't, walk away. I would say that they should be around $80 from the dealer, but should be around $60 from an AC Delco retailer if you have one close by.
That is the last advice I'll give in this thread unless I start getting actually listened to. I hate repeating myself and will not do it again in this thread. I'm trying to help you save money and get the problem fixed if it really is a problem, but if what I suggest isn't done, well... Your loss.
Reset the code first and foremost.
If it comes back, check out ACDelco.com
and see if you have any parts stores close. If you do, that is your cheapest, fastest, and best bet for any of these components.
I picked up 2 AFS105 sensors for a bit under $130 from the local AC Delco retail parts store when I had my Bravada.