PCAVENDER, my suggestion is if its an 08, take it to the dealership and explain your situation, im assuming your still under warranty. But we all know what assuming does......Anyway, there are too many components that can fail to give you this scenerio that i cannot give you any information except, if you really want to figure it out before you take it in (if under warranty) get a good vacuum guage and go through the testing procedures and pin point what has failed. My guess as to what happened is that whatever was causing the vacuum leak and giving you the grinding noise finally gave out, allowing the wheel ends to engage (the clunk) and now they wont disengage, giving you bad gas mileage because you truck is now having to push the front rotating parts. This is not a bad scenerio to be in if the fronts stay locked in because the front is now locked and not trying to lock and unlock all the time, grinding, and wearing the gear on the front axle ends. So as long as you dont hear the grinding the worse thing that is happening is your bad mileage until you get it to the dealer.
If your out of warranty and able to work on your vehicles with the appropriate tools, i would still get a good vacuum guage and pinpoint the problem using the methods on my first post in this thread. Then you will know what has happened and what is then needed to repair it.