Yea, there is nothing wrong with the 3.0 pushing big tires. you will spend more money swapping the 4.0 in because you will need wiring harness, tranny, the whole motor. It won't be cheap.
Might as well spend dome money on bolt-ons and get almost the same hp as a stcok 4.0 for a lot cheaper.
My 3.0 pushed 33's easily with a few add-on's like intake, exhaust, programmer and still got damn good gas mileage.