You have to watch when they change injectors. Part of the time they don't get them setup properly, and your mileage SUCKS!! I have an 06 3/4 ton, with right at 150,000 miles on it. Up until 125,000 We never put a dime into it, the last 25,000 he had to put a front cover seal in, both rear wheel seals, and a rear out put seal on the transfer case.
I love ours, and running empty, with out a trailer, I can get nearly 25 mpg. With a trailer, and 14 big cows on, I am down to 13 mpg, but I can out pull most of the rest.
I am thinking about pulling the catyletic converts off, and reprogramming it. I think I could get upwards to 30 mpg, maybe. Other thing I would add would be a Jake Brake!!!