I read something in the owners manual that says this is normal... the car stays in 3rd gear till it warms up.

Can this switch go bad? Sometimes my truck likes to jump back into 3rd... like when i was coming home from LA doing 80mph down the grapevine.

Can you override this? I live right near the freeway, and use it to get everywhere, so this feature is especially annoying.

The sensor could go bad, or the ECT module may have a problem, for it to downshift @ 80 MPH.
You could probably bypass the sensor for the warm up circuit, but it may be used for other things too. You'd have to look at a wiring diagram to be sure.

See my thread on "funny symptoms"
Yes, their are 5 temperature sensors in the back of the intake manifold, kinda hard to get to, but one of mine, or more, failed and made the truck awful to drive. I replaced the ones that feed to the tranny, ect, and fuel injection (yes I know that doesn't make sense, but a more detailed explanation I cannot do from the top of my bald head).

The dealer in utah replaced the head gaskets, thinking thats why it ran crappy, and it was no better after they did it.

Don't override this. Replace the "ECT start timer sensor" at least and your problem will be fixed. I think that one was $125 from the dealer, but the truck runs like new. Read my other thread for lots more detail.