Do a search for a thread titled "high mileage Explorers." Theres a ton of info in there about repairs, associated costs, etc. Personally, I think the best component in the truck is the engine, especially the OHV V6 (time will tell about the SOHC, but aside from the timing chain issues, they seem to be holding up pretty well). The OHV though is almost bulletproof. I have over 150K on mine with zero engine work done to it, aside from the usual plugs, oil, filter changes, etc. I think the only big thing to go wrong was the valve cover gaskets were leaking and needed replacing. Just make sure you don't overheat it or you'll crack the heads. Besides that, the tranny, as mentioned, is a big trouble area. Mine went at 95K, and the replacement is still holding up fine. I hate to say it, but after 80K was when the big things started to go wrong. There was a lot of A/C work done (I think the only original part of the system is the compressor), a new alternator, new power steering pump, new master cylinder, and some suspension work done. Plus all the usual wear items like pads, rotors, shocks, tires, a new battery, etc. Actually, now that I look at it, that's not that much, just big bills associated with them.

That's to be expected though. I mean, how many people these days really keep their cars anywhere close to 100K, let alone 150K (I'm the original owner). A little frustrating at times, but things do wear out eventually. Overall though, they're good solid vehicles, and to tell you the truth, I never expected mine to last as long as it has, and still look and drive as good (or better) than new.
Mike