Are you really going to let them off so easily? I would be even more motivated to get some satisfaction if they did that to me. They are hoping you will fold and it appears they were successful with very little effort on their part. It's your truck and your battle but I hope I can convince you to try one more thing.
You need to go to the shop GM and ask him who his boss is. He may start backpedalling immediately. If not, then go speak to that person. Be calm but firm. Tell that person what happened and that you feel they are trying to duck responsibility. Explain that you attempted to be reasonable and only wanted assurance that no damage was done.
They could fix this with very little expense by purchasing the manufacturer's extended warranty for you. If not, you've already been advised to take it elsewhere for a teardown to assess the damage. You will be seeking full restitution for all diagnostic and repair work. You've spoken to a local independent mechanic who is ready and willing to not only do the work but to represent you as an expert witness if the need arises.
It would be best for both of us to come to an agreement and avoid all of this so you'll give them until tomorrow afternoon to decide to do the right thing. I'd suggest that he consult with his legal counsel regarding his potential liability and the financial wisdom of offering up the warranty to avoid protracted litigation and expense.