$3k should be plenty of money to buy a good one.
Remember, the used-car market has changed drastically in the past couple of years. SUV's, especially the larger ones are now as popular as plutonium. Nobody wants them (except you and me it seems) so prices are depressed. Sometimes, people will ask a relatively high price, but its rare that they will get it. If you look around a bit you can find someone who is relatively desperate to sell and is willing to take a low offer.
Last summers cash trade-in program did harm the used-car market a bit by removing many decent used cars from the market, thus driving up prices a bit. But as time goes by, prices will realign to a more reasonable value.
One more thing.... why do you specifically want an Exploder?
When I go shopping for an old, cheaper car I look for the best possible dea (ie: best condition and lowest mileage for the money)l, and not for any specific brand. So, perhaps you might want to keep an eye out for cheaper Blazers, Cheokees etc, and not just the Explorer (although imho I like the explorer best)
Also, look here:
www.carsurvey.org. It's an excellent site for getting a handle on some common problem areas for these vehicles.