1999 Ford Taurus Speed slumps after letting off the pedal

Thanks for all your help (I read your site all the time) and I will try to explain this problem as best I can.

When I get the car up to speed on the highway (50+ mph), if I let off the gas the car will coast for one second and then feel like I am lightly applying the brake. Instead of coasting smoothly the speed slumps noticeably after a second, you can even feel a soft lurch when it happens.

It is the 3.0L V6 OHV ("Vulcan") engine with the AX4S transmission.

Any idea what could cause this?

Thanks very much!