Didn't this year come with 4.10's on the 4 cy? Then rpm of 2500 isn't actually off and 31's wouldn't make any difference. When was the last tune up? try putting some injector cleaner in the gas the next filled up and for the next every second full up for a few times, what does the air filter look like, try the simple things first and get back on here and let us know if theres a little improvement. Then what about codes, are there any?