I'm going through the same thing with my 2005. I spent almost 10,000 on it last year. The brakes locked up and it cost $1200 and the head gasket blew and that was $8000. I thought about trading it in, but I wouldn't have gotten much for it with the blown head gasket and a new truck comparable to what I have costs $52,000. I'm working on a year-to-year contract and couldn't swing payments on a new truck if my contract isn't renewed every year for the next five years.
The military replaced a vehicle when the cost of operations exceeded 50% of the replacement cost. Using that standard I still have a ways to go. However, there's a lot to be said for the piece of mind that comes with owning a new vehicle that's under warrantee.