I picked up a couple MPG when I swapped in the 4.0, then I lost those MPG when I put the 37's on. So I'm back to the mileage I was getting with the 3.0 and 35's which is around 18mpg.
I wasn't too dissappointed with the power loss. Gets around fine on the road. I'm used to it though, the truck has always been a turd.
Single low 2nd gear (hubs unlocked) will still spin the 37's in the dirt, but it doesn't rip the tires off like it did on the 35's.
If I go to a larger tire I'll definitely regear. Other than that, I can probably live with the 4.56. 5.13 would be recommended for someone going straight to 37's.
I weighed my truck the other day too. Full toolbox, full doors on, and me in the driver's seat I weighed in right around 4500lbs, just FYI.
edit: I should add that I'm running the 3.0 m5od trans internals in my 4.0 m5od trans housing. That gives me a lower 1st-3rd gear than the other 5 speed 4.0's on the street.