his JK has no bottom end power, its actually kind of pathetic. 6th gear is only suitable for very flat roads, hit the smallest hill and you have to down shift.. that leads me to ask, what did they change in the D30? on my XJ the biggest you can go is 4.56 because the housing is just too small for anything else.
The D30 can go with 5:38's, although once at 4:88's, the pinion is pretty small and prone to problems.
The D35 rear axle used in Jeeps,
used to be able to only go to 4:56 (you can now go to 4:88's through Superior Gear)
That may explain why you were told your Jeep could only go to 4:56? If they were taking both axles into consideration.
The '07 to '11 JK's are under powered and doggy, the 'drive by wire' throttle does not help any.
Regearing to lower gears (higher numerically) will bring the engine back closer to it's 'power band', and tuner chips, CAI with bigger exhuast,, will help with horsepower.