All of a sudden, my Explorer decided it won't idle any more. I can start it just fine, but it dies as soon as I take my foot off the gas. Any idea what might be going on?
You either have a major vacuum leak at the hose to the brake booster where it connects to the engine, or the intake gasket. A diagnostic inspection of your stalling condition by a mechanic, like one from YourMechanic, will verify the leak location, and you will get a quote for the cost of the necessary repairs.
You can check for the leak with a piece of hose and listen around on the engine and brake booster and listen for the leak. Once the leak is identified, then a mechanic can come and service or repair it for you.