I drove my 2004 Ford Ranger 3.0L V6 on the highway at 120 kph (65 mph) in second gear, and the engine died. Is it seized or can I repair it? The transmission fluid is still the proper color and doesn't look burnt.
If you can turn the engine over using the starter, the engine is not seized. Just because it died after such an event, doesn't mean it is going to do that. Definitely not good for the engine, but it doesn't mean it's seized.
It really depends on how the engine died to tell you what exactly happened. If it just quit without any loud bang or loss of engine fluids, then it could possibly have seized, but still not a high probability of this. If there was a loud bang with loss of oil all over the road, that could be a thrown rod. I doubt this happened, otherwise you'd have mentioned it (kinda hard to miss an oil spill the size of what the Exxon Valdez left behind).
Even if an engine seizes, it doesn't mean you've killed the engine. When an engine seizes, what happens most of the time is the rings grow in size until the point where the end gap is gone, then they grow a little more and stick in the bore. When they cool down, they release and the engine will run fine again. This doesn't mean it doesn't cause damage or wear and tear, because it does.
If none of this pans out, I'd suggest you have a different problem. You may be able to put a code reader on the OBD-II port (DLC) and get a read on what's going on.
Next time, leave it in drive ... makes for a much happier engine.
If it just died without any loud noises or the wheels locking up, and still cranks by key or by hand, then it's almost definitely not seized.
It could be a number of things, like timing belt, lack of fuel/spark, or sensor failure. As suggested, connecting it to diagnostic computer may be helpful to point you in the right direction.