As the others say, 2.5 is fine offroad, My 2.5 YJ disgraced many a 4.0 in the gooey stuff. On the blacktop, you do have to push them harder to keep up, negating any chance you had at saving fuel and making driving a bit less relaxed. Unless you can get one silly cheap, you're better off going 4.0. I wouldn't consider converting a 2,5 to 4.0 as it would cost loads more in money and hassle than selling it and getting a proper factory 4.0 instead.