smoke, odor, headache

[Hoopty]

I've got a 97 Explorer Sport 4x4 with 4.0 OHV and 5r55e. At 98K miles, it's been pretty reliable until today. I noticed a sudden odor of what smelled like burning plastic while driving on a country road. I saw no smoke inside or out so I pulled over near a church to find an extingusher, if needed, and to pray. I opened the hood expecting to see the worst, but no smoke or fire. The odor was definately under the hood. I checked around, started and stopped it, and started out again after finding nothing wrong. I got back home and only once noticed a puff of black smoke. I'm almost sure it was from the exhaust.
I checked it over again and the odor seemed more like coolant. I didn't see any obvious leaks in the system including heads, block, hoses or heater core. All fluid levels are fine. No C/E light either. Nothing seems wrong now, but I'm feeling less secure about this thing. Any ideas/experiences?

 

[TPLYNCH]

Check to make sure the oil drain plug isn't leaking. Mine did that, dripped oil on the exhaust. Also check and see if maybe a plastic bag got caught under there.