I have a 2000 durango 4x4 with 47k on it. Recently I've noticed that my engine will rev from around 2000-2400 rpm when I'm going upwards of 45 mph. It feels like its shifting between gears, even in cruise control I cant get the rmp's to "mellow out" and stay on a certain rpm reading.
On occasion, the check engine light will come on.
Any help on what's going wrong would be greatly appreciated

Sounds like a speed control sensor problem. Speed sensor cost something like $20-$20 bucks and located near the rear of the tranny and can be easily changed out yourself if you have some basic mechanical knowhow.

Throttle position sensor is bad. You can pick one up at any parts store for $25. Just unplug it and 2 screws to remove it from the drivers side of the throttle body.

danny