We first noticed a problem when our 98 expedition started overheating, we put in the coolant and went on our way. A few days later smoke started coming in the vents when the AC was on high. We checked the coolant and it was almost empty. It's been going through coolant like crazy but we don't really notice big puddles under the car, it's mostly when we are driving it I guess. The smoked smelled like coolant and comes back anytime the coolant levels gets real low. Any ideas??

Could be your heater core leaking. Try some engine coolant "stop leak" in the radiator. Be sure to have the heat turned on when you do it, so the coolant and the rad stop will flow through the heater core for a while. I have the best luck with the stuff that is powdered aluminum . Pour it in your rad when it is low, and then fill to level. If that doesn't do it you may have to replace the heater core. Or, it could be just a hose leaking under the hood, and the fan is sucking it into the truck. Inspect all hoses and connections and gaskets closely .
It probably just drips until the coolant level get's low, and then steams forcing you to smell it.

Your auto is definitely loosing coolant and will overheat. Check your thermostat, hoses, belts, radiator cap and water pump. If you do not fix the leak or coolant circulation problem you can destroy your engine. The smell inside can be being picked up from the outside vent or it could be your heater core inside the ventillation area leaking (which could be your coolant leak).. If that is the case you will need to replace the heater core.