I've never driven a 4.0L more than 15 miles, but my 2.5L cruises the highway at about 3 grand (taller tires, stock gears, 3 speed auto), and for passing traffic's sake, the truck seems to be revved up to about 4500 before shifting in town.
I don't necessarily consider this high RPMs, but it's certainly not the down-low grunt work in which many of you are referring too.
Right now, I'm running the stock exhaust system with a 2.5" Magnaflow for a muff, so maybe the Magnaflow is the issue and reason I have to shift at such high engine speeds, but I don't believe a muffler alone will make that huge of a difference.