I am not trading off my Dart! Need a new truck.

It is the spark plugs, changing them is about 2 grand and should only be done by a dealer. The engine has to be dismounted and moved to clear the rear ones. Just like the old mustang 427 and 428s. Pretty much what Wildcat said.
I would always get the biggest best engine and for my money it is the 6.2 LS in that Denali. I love the Hemis but you really almost have a corvette engine in that truck.
Best of luck though but i recommend the Denali, you know what they are like, then the Ram. Nothing ford unless it is an 03 or less Powerstroke 7.3, or the newer 6.2 gas or 6.7 diesels. and dont get me started on those eco boosts lol.