I just bought 2 of ebay for 60 bucks and i was wondering if i should replace the one behind the cat or just save this one it i need it for the front.

 

I would save it, also the o2 sensor can be taken off and cleaned with a soft brass brush and some carb cleaner. Just make sure you blow the sensor clean with air. By doing this you may buy yourself another 50,000 miles. I have done this with a great deal of luck.