[shorod]

What does the "smoke" look and smell like? Is the smoke white, or does it have a blue tint to it?

There's a chance all you're seeing is condensation that's puddled in the exhaust system from the humid weather that's boiling off when you start up your Explorer. If the "smoke" doesn't have a sweet or oil smell to it, it's probably just condensate. If it smells like oil and has a bluish/gray appearance to it, valve stem seals are probably the culprit like rhandwor suggests.

-Rod