[crazypete]

Ponyexpress:

The torque converter IS the fluid coupling. Of course, the exact same problem can be caused by an old AOD with worn clutches. The clutches grind down and then slip all the time, devastating performance and mileage at the same time. The only way to tell the two apart is that a bad converter will trash your city mileage and leave your highway milage the same but bad clutches will give you across the board terrible mileage. The guys at the local yard said that if you burn clutches long enough (like not hooking up your TV cable), the car will basically neutral all the time since nothing is able to transmit power to anything else. Rebuilding an AOD is easy enough but it is messy and that damn thing is HEAVY. I rebuilt an AOD on my first try never even having seen one before I bought a used one from a yard. It's just stacking things up and dealing with a few snaprings. Rebuild kit is less than $100.

Tincan:

Is your highway milage (or performance in 3rd/4rth) much better than 1rst/2nd and city mileage. That is a sign that it IS your TC. If it sucks all the way around, I would say it is the clutches in the transmission. Since it doesnt vibrate on the highway, I would say that it is most likely the TC since you have a direct connection bypassing the TC sorta in 3rd and totally in 4rth.