Most likely the torque converter. The splines for the oil pump drive shaft strip out causing no line pressure and therefore no engagement of the transmission in any gear. Fixed a lot of Windstars for this problem.

About the head gasket... If it's the OHV Vulcan 3.0L V6, it's more likely to be a cracked cylinder head. They tend to crack between the valve seats. New heads from the dealer are actually pretty cheap, at least they were the last time I checked (I think it was $250 for a new complete head with valves and springs).

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Erich Loepke
2010 Ford Focus
Currently Benz-less