I have a 2000 Dakota SLT with 63K on it and have noticed that it isn't shifting into gear when it reaches right around 40MPH. It will stay in the wrong gear until between 350 and 400 RPMs. Do I need to replace the whole transmission or could it be just a sensor?

check your fluid level to see if it's at the right level. Assuming you have a V8, sounds like your overdrive band is slipping due to the fact that it's worn out or your fluid pressure is low due to low fluid in the tranny.

That was it!!! I flushed and refilled the fluid and she runs like a drem. BTW its a V6.