Is the added cost of a new truck going to be outweighed by getting a few mpg better? Not to rain on your parade, just wanted to make sure you thought about that. A lot of people jumped on the hybrid bandwagon for no reason other than because they could, now they're screwed.
Even if you found a truck that got 5mpg better,(which probably would be hard to do consistently w/ a full size truck) over 120miles each day you'd pay for 9.2 gallons in your truck, or 6.6 gallons in a new truck getting 18mpg, so saving you roughly 2.5 gallons of gas each day, or about $6.25/day. If you drove 5 days a week, for a month, that would add up to $125/month, can you get a newer, better mpg truck for that much or less, without losing money selling your current truck?
Didn't mean to ramble, sorry. Just my .02