I lost a head gasket in my 05-5500. Mostly due to operator error (employee), they do have upgraded gaskets for this year of Dmax. Pissed yes, lightened my wallet by 4K. GM did help me out some. The truck had around 50K on it (the warranty on the med. duty is not the same as the lighter duty Dmax trucks). I also don't believe the engine is permanently seized, not given the fact pattern that you gave. Still likely expensive to fix, A short block is not that expensive worse case. Should it have happened, certainly not. The upside is there is not a diesel pickup that holds its value like a Dmax. So the sting of repair can somewhat be lessened knowing that your not throwing money down a hole as you do with a Ford 6.0 I would get the pickup looked at and explore your options, there are some great Dmax websites for tons of information. I would consider replacing the LLY with an LBZ.
