Sounds to me like water pump isn’t doing it’s job full time. You said you replaced the alternator in your other post, did you ensure your belt is routed correctly and your tensioner is keeping tension?
Other than your recent highway usage, are you a stop and go short trip driver or do you have a decent commute to work? I’d get a scan tool reader and check to see what code your getting
I wonder if the line joints are failing, not giving the injector enough pressure to fully do its job... deff interesting it’s leaving spots under the truck. Times like this makes me wish everyone lived closer so we could team up and check out these issues and brainstorm.
If you can get ahold of a code reader, that will better direct you to a solution. Modern vehicles are a lot more sophisticated than before and normally the code will pinpoint you to the correct fix. You can replace the entire emissions system for around 5-8k $ but if it’s just the def injector...
Maybe the splined end for the pumpkin comes out? I noticed mine have a lot of play when they go into the front diff but I don’t have any odd noises or clicking lol