Hi YJSteve, everyone has given you great information but I noticed there hasn't been a basic explanation, unless I missed it.
If you park your jeep on a flat level surface, release the emergency brake, chock the front wheels and jack up one rear tire off the ground then you can see what's happening. (You don't have to do this, but you can if you want.)
Let's say your rear end is the standard 4:10 gears. If you rotate the wheel one full turn you'll see that the drive shaft rotates 4.1 full turns. If you have 3.08 gears then the drive shaft will rotate 3.08 full turns. Pretty simple.
Now, let's assume that the gear ratio in the transmission relative to engine RPMs is one to one (1:1). This typically the standard ratio for manual transmissions in 4th gear. The 4:10 will cause higher engine RPM (and generally higher horsepower and higher torque) than the 3.08 at any given highway speed, say 65. So, if you're running 65 with a 3.08 your engine (1:1 ratio) is operating at low range in the RPM/HP/Torque curves for the engine and there is less "power" available than higher in the RPM/HP/Torque curves which is made available by the 4:11.
Jeep already selected the optimum ratio for you considering both power and economy. As stated before if you start modifying things (bigger tires, more weight, etc.) then you might have to consider different gear ratios depending on the mods and the HP/Torque curves for the engine.
Here's a link for the curves on the 4.2 litre engine. Sorry I dont have them for the 2.5 (they're entirely different) but this will let you see what's happening:
Hope this helps, and take care.