I am new to Jeeps, I am building my first one, but if you will take a Ford persons advice...
Gear Ratio - jack up the rear, the whole rear.
Mark your drive shaft position on the axle housing.
Mark your tire for top position.
Turn the tire around for a full turn while counting the number of times the drive shaft turns.
If the driveshaft turn four times to the tires once, your gearing is approx. 4 to 1, get it...
My thought on target rpm is fourth gear, 2600 rpms, 60 mph.
I will be running 4.56s, 35" tire, and my calculations come up with 2680 rpm in fourth at 60 mph.
Gearing is based on tire size vs rpm...
Find out your existing gear ratio and tire size, i.e. 32", 35".
If the tires are not the originals and the gearing is, find out the original tire size.
Decide what tire size you want to go to, 3s or 35s maybe.
Divide the old tire size into the new tire size and multiply the answer by you existing gear ratio.
What will that give you, the new gear ratio you need to run to increase the performance of your Jeep.
I.e. - I was running 4.10 gears with 33 in tires, I want to run 37s, run the formula - New tire / Old tire = ratio of change ( 37 / 33 = 1.12). Take the ratio of change (1.12) and multi the existing gear ratio (4.10 x 1.12 = 4.592) So I should be running 4.59s, no such thing so I can go with 4.56s or go higher with the gearing to get a little more power from the engine.
When you do run the numbers and find out your gearing remember that you had an under power condition before so go a little higher in the gearing to get a little more power feeling.
As you can see the process isn't hard just take a little time and decide on the changes you want and plug them in.
Also, there are sites that have calculators to determine all of this just by plugging in some numbers. Where ??? Sorry not sure.
Work/Cost of re-gearing - To re-gear you will need to disassembly both differentials put on the gears and install the pinions. If lucky the diff won't give you too much trouble coming out and the pinion/diff shims will be the same as the old shims.
Mt thoughts would be if you don't have much experience with this type of work leave it to someone that has, it will pay off in the long run. The cost should run approx. $750 for both diffs, including gears and new bearing/seal.
Sorry for the long response, I hope you can understand my thoughts, I tend to ramble some.