Open a separate thread for your issue so you'll get more feedback.
I've heard this before... Sounds like an aggressive dealer who wants to cover his ass at your expense. You still have bumper to bumper warranty so you should not accept ANY risk for warranty work. You've followed the maintenance recommendations, have oil change receipts, no mods done to truck - so you didn't do anything wrong and they can't give you any of these bs conditions. What if they'll have an inexperienced or lazy mechanic tear it down, and he feels like nothing is wrong with the engine, and puts it back together .. Do you have to pay labor of a few grand? Doesn't work like this during bumper to bumper warranty
I'd argue with them the following way:
1. Your engine sounds bad and abnormal as if something is broken inside.
2. Since it sounds very bad and abnormal, the engine is not operating properly and has a problem inside of it. They agree that it has a problem since they're recommending a tear down. Otherwise, they would just tell you it's normal and go away...
3. It's their job to fix problems in your truck at no cost to you while your warranty is in effect and you have proof or performing the required maintenance as per manual
Check reviews of this dealer to make sure they got competent mechanics. You can also check out another dealership if this one digs in. They're supposed to take care of you as you fit full warranty coverage.
It's not good to tear down a new engine that early, but unfortunately you don't have much choice besides selling the truck or ignoring it until engine blows up. Although if repair is done properly then it should be fine. Research dealerships in your area, check reviews, this is a big job and you want to make sure you take it to the dealer with most experienced techs in your area. My truck has continuous issues for few years because dealer did a poor job.
Keep us posted.