Hey Ted, I had a 98 Windstar that did this. I would drive, come up to a stop, go to accelerate again, and it would just rev.
It was an internal trans problem. Neutral sensor switch. We never found out if it was on the computer end, or the sensor end cause the head gasket blew out first.
If you put it in park, shut it off, and turn it back on, does it go into gear?