So in 2002 or 2003 the dakota stopped using 2 modules for the engine and trans and only have one PCM that controls both the engine and transmission. The P0700 is litteraly just a pcm code that tells you there are more transmission codes and you need a more advanced code reader to get into those a basic one won't do. Even though its all in the same computer its in a different section in the programming.
There's also a demi module IN the transmission on the solenoid pack. Which is most likely the issue. Mine did exactly what you are describing. Ended up being a solenoid in the trans going bad and it kept going into limp mode. Common issue in the 2003 and newer dakotas/durangos. The solenoid gets weak?? for lack of a better term and lets the clutch pack slip and you get a improper gear ratio code and trans goes into limp. The solenoids on the 545rfe are an all in one type of thing. Mine ran over $300.00