3.9 is a gas hog and weak! I would go 4.7 and do my best at maintaining it. My quad cab 4x4 gets 20-21 mpg on the highway with a drop in K&N, super chips programmer and a 02 Jeep Grand Cherokee TCM so I have the 5th gear in the transmission. I have seen a pile of 3.9's with turned rod bearings so they aren't very tough either. The main thing is how it was treated!