Bring your truck over to Autozone and they'll check the code on the engine light for free. As for if you'd be responsible for something they did... well, they have you sign all sorts of **** that probably covers them of all reliability. Before you worry to much about it i would just check the code and see what the problem actually is.
Also, you usually can get a much better price from an individual/shop rather than the dealer. If the dealer is estimating $758, I'm betting you can get it done at a lot better price elsewhere. HOWEVER, if you're truck is still under warranty than it might void the warranty, so you might not have a choice. Not an expert myself, but I would at least start by figuring out which part it is that needs to be replaced then find its price online. Just my recommendation though.
Best of luck, and welcome to the site!