Should I trade my 5.9L for a 6.7L?

[JSchutz]

I work six days a week and emergencies on Sunday (large animal veterinarian) so reliability is my #1 requirement in a vehicle.

My present truck is an ‘03 with 264,000 miles (yeah, 65k/year). The truck has been nearly perfect--only a few minor repairs (one wheel bearing, one u-joint, throttle position sensor). Computer reads 21.8 mpg over last ~10 tanks. (my driving is mostly 45-60 mph on country roads leaving me locked up in OD at 1500-1800 rpm--seems to really maximize the fuel economy. If I reset and run on the interstate it drops a couple of mpg).

My dilemma: I can trade my present truck while it still has some value (dealer((Big O, Greenville SC)) is offering me $12k for the trade, ~$37k for a new 6.7L similarly equipped=$25k total deal) or drive it a couple more years after which it will be over 400,000 miles and basically be worth scrap metal price. Driving it two more years will thus cost me $500 per month in depreciation, probably only slightly less than the depreciation of a new vehicle. The old one runs great, but for how long? Pretty soon things are going to have to start wearing out (I still have my original ball joints and lift pump!) I’d love to see how long this one would run but if it goes in the shop I’m stuck throwing some tools in a beater car and pretending to be a vet. On the other hand how reliable are the new trucks? Is it realistic to expect 200,000 trouble free miles from one of them? Perhaps I should trade every year or two but it takes nearly 75,000 miles for the fuel economy to max out and I hate to trade so soon after it gets good.

What do you think--stay the course or trade? Jeff