1996 Ford Explorer revving up

209,000 first thing is it probably needs a new clutch, but peek under it at the drivers side of the tranny, any sign of wetness? The slave cylinder, or even the master could be defective. Second, since you dont know when the last tune up was, its a good idea to at least pull the plugs and have a look. Last the revving could be caused by a vacuum leak. Check it this way. You may have a vacuum leak, get a can of spray Throttle body cleaner, spray the vac lines and maniflod area, when the idle changes, youve found the leak! You want the engine running and spray on the outside but do direct the stream onto the hoses, if it is a vacuum leak, the engine idle will change speed, then you have detected a leak, repair the vacuum leak and see how it runs! If that doesnt do it, you may need a new idle air control valve, it's bolted to the throttlebody
If the check engine light is on, have the codes read, Autozone does it free, post the codes here.