Re: 95 Explorer XLT O2 Sensor ?
I'd wait. I have an Aerostar with 160K miles on it and it's on the first one, my Escort's went out around 120K, my Focus's went out at about 70K.
When either one goes, the Check Engine Light usually comes on. The code will tell you which it is. Depending which one goes out, when it finally croaks your truck will either run like crap and get worse gas mileage than usual (pre-cat HO2S), or the check engine light will just glow and nothing else will apparently be wrong.
The upstream (Pre-cat) O2 Sensor is used for tuning the Air/Fuel mixture. The downstream (Post-cat) is just used to test the catalytic converter. With the exception of the Altima my dad used to drive, no car I've ever worked on or owned has had the post-cat O2 Sensor go out.
Basically, if you want to replace it, go ahead, but make sure you get a genuine motorcraft sensor. I know they're more expensive, but I've had nothing but problems with the botch errr I mean Bosch sensors from places like Advance Auro Parts or AutoZone.