Hard to say what it is, but everyone is correct, wheel balance is the simplest way to start. The second thing that comes to mind are all the other 4000 parts in the front end.
3rd gen Dakotas have goofy front ends, and they tend to vibrate from all sorts of maladies. The brake rotors (disks) are the most likely culprit to vibration under braking at highway speeds, but next, I'd have them check outer tie rod ends first, as they do wear out sooner than most flexible Y stud type assemblies. They can give a false positive too if the mechanic doesn't push hard enough on it with a pry bar.