I'm sure your transmission is ok.
The P0171 and P0174 indicate that your engine is running lean, thus you've 95% of chance to have a
vacuum leak on your engine. It can also be a bad/dirt MAF sensor.
A severe vacuum leak or a bad MAF can cause many symptoms like lack of power, hesistation, vibration when changing gear, misfires and many other symptoms.
You can read more info on vacuum leak here:
http://www.aa1car.com/library/vacleak.htm
Many things can cause misfire, including a vacuum leak. The misfire cause the check engine to flash.
On most Windstar years, transmission is programmed to shift from 2 to 1 when you brake to stop and cause engine vibration because of compression and engine brake. When engine has a vacuum leak, vibration is more higher, because you have more air in cylinder (thus more compression) with not enough fuel to burn, so the engine brake the van instead of accelerate it when it downshift w/o gas.
I remember when I had the 171/174 codes in one of my previous Windstar, when I floored the gas, the van almost no accelerates. I mean it's like if the power was limited to 100HP or something.
So my point is your vacuum leak or bad MAF is causing these symptoms, not the transmission.
Change fluid will fix nothing. If fluid is pink dark, It's too late to flush it and can kill your transmission if you flush it. And check fluid only when it's hot and engine is running on an horizontal surface. If level is too high, it can damage the transmission.