Wow. Innocent questions with big answers.
The first thing you will want t do is figure out what you want to do with the vehicle.
Is the gearing change just to make up for bigger tires on the highway?
Or do you want to offroad with it?
Or do you want to rock crawl with it?
Different questions requiring different gear ratios. You can get an idea using a gear calculator that can be found all over the net. Lots of sites have their own. You can plug in the stock gear ratios and then experiment from there. Here is a link to our calc page at iZook.
And here is a link to the gearing tables that list most of the ratios that are out there.
Old school thought was that if you only needed to adjust to 31's, you could get a 4.16:1 (GRS1) to do the job. If you wanted to go to 33's you would want a 4.89:1 (GRS2). But you can get more power (with a slower top speed)
with a 5.1 - or 5.4 - or 5.8 - or 6.4 - or even 8:1 gearset.
If you are not comfortable with ripping apart your t-case, you can get complete units from Petroworks, Trail Tough or Low Range Offroad. They will also give you some pretty good advice on what you need for your situation.
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