If it is 4x4 your looking at changing both ends. It is NOT a DIY item unless you have LOTS of practice with gearing. All in your probably looking at between 2000.00 and YES the computer does need to be reprogrammed, it controls much more than just the speedometer. Shift points, engine management and the ABS system are slightly different depending on gear ratio.
As to the current ratio, look in the glove box for the options sticker. On it look for a code that has GU in it and post it here. That will tell you what the gearing is. Keep in mind that just because you increase the gearing that does NOT increase the braking ability. Make sure your brakes are rated for the higher amount first or you might be better off trading up. The brake capacity is also on that option code sheet.