After thinking about your situation more, I really think you should stand your ground. Your truck is paid for, 1 year old, and still under warranty. Why on earth would you want to trade it in and pay 18K more. Furthermore, Ram stating that they will cut off your loaner vehicle is even bigger BS. They have your work truck, and you get nothing? What would happen if June rolls around, and Ram says, "Sorry, the backorder has been pushed back to 2018." Are you supposed to not have a vehicle?
I would tell the dealer to get bent, and then I would start making calls. BBB, lemon, Ram, dealer mgr, etc. The bottom line is, until 3 weeks ago, you had a fully paid truck, that is under warranty, that you were daily driving. Now, they have taken it away from you and are holding your truck hostage. Either demand they give you a loaner for the entirety of this debacle, or demand they trade you another 2016 6.4 with comparable miles at no charge, or a 2016 6.7 with comparable miles for the price of the 6.7 upgrade.
There is no way I would pay 18K when this whole mess is their doing.