Torque is all about cubic inches.
It takes a lot of "up-grades" to get the 4.0 to equal a 5.7.
Stock 5.7: (carburated) 9.5:1 compression; 343 FT Lbs at 1750 RPM, 375 Ft Lbs at 3500 RPM, 342 Ft Lbs at 4250.
Stock '06 4.0: (MPFI), 120 Ft Lbs at 1600 RPM, 235 Ft Lbs at 3200 RPM.
My stock 4.0l couldn't hold a candle to my stock 5.7.
The 4.0L is just fine for rock crawling, and cruising unloaded. Add all the weight from a moderately built rig, and, "poof", there goes on-road performance and gas mileage, especially when loaded down for camping.
As I added toys to my XJ, and the weight went up, my mileage dropped from ~22 to 16!!
As far as I'm concerned, I now have my cake and eat it too.
No more grinding up Calif mountain grades in the "truck" lane--now I can run with the "big boys", and still get 20+.
The swap wasn't easy, and, considering the cost of a second engine, it wasn't cheap.
Of course, Chrysler was all wet putting the 318 and 360 in the Grands.