O2 Sensor

I was told by an expert mechanic and also read from auto web sites that the Oxygen Sensors tend to deteriorate after the vehicle has had some mileage. That although the vehicle may feel like its working OK, that a contaminated sensor can become sluggish and unresponsive. Some experts recommend replacing high-mileage O2 sensors to restore like new engine performance.

What's your take on this?

Yep, they do become bad after many many miles, even if you let your vehicle idle a couple minutes a day to warm up or whatever that adds up too. replacing that can make a difference but aren't a cheap thing to buy.

My 99 Ranger 4.0 has two O2s (one on each bank upstream) and one after the cat converter. Because of cost I would like to replace them one at a time however, it may be necessary to replace the pair of upstream sensors together. Think so?

If it's not absolutely necessary I would do one at a time. If your truck isn't running to bad just do them one at a time but if you think it should get both done then do it that way.