DEF tank needs to be replaced!? The DEF tank is likely made out of polyethelene or similar plastic. Unless the DEF fuel is contaminated with silicone I really doubt a DEF tank can go bad. Or is there something integral to the DEF tank that requires it be replaced in total. Perhaps the DEF tank heater is integral to the tank? I could see a tank heater failing.

With the cost of diesel going far more than gasoline, and the fact you have to buy DEF for it as well, if you can get a good trade value for it I'd go to a gas engine. Those are WAY too many issues for a truck, that to me, for the miles is nearly new.