The transfer case has nothing to do with gear ratio. If you simply want to change your gear ratio, you need to change your front and rear differentials.
Do the math on your fuel savings. If dropping down to 3.08 gears lets you go from 19 mpg to 21 mpg, if you drive 20,000 miles a year, that’s a savings of about 100 gals. At $4.00/gal that’s only $400 a year. If you spend $1000 or more on the conversion, you won’t see any savings for almost 3 years. If you drive fewer miles; the longer it will take to pay yourself back.