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IIRC, our Ds have 4 sensors. 2 pre-cat and 2 post-cat. Relatively easy to take out. Just make sure the exhaust is cold or you may burn yourself! I'm sure others will chime in when they wake up.
the 4.7s have 4 as dan said, the older 5.x series engines have 2, if you are not getting bad gas mileage, or throwing a cel, i wouldnt change them as they are expensive, and if your mpgs drop only change the pre-cat(engine side) as they are there to control fuel mixture in warmed up conditions
i have never had to change mine and i have 90k on my truck (yes MILES) is there some reason you want to replace them? if you share with us the problem we will most likely have a cheaper alternative, as they tend to run in the $80 price range (each) if i remember correctly from what others have spent