Gear choice will be dependent upon original gear ratio and tire vs what your new tire size will be. There are a few calculators on the web. I used this one http://www.ramblerman.com/blazer/gearcalc.htm. For me with a 4cyl and stock 4.10 i had to go all the way to 4.88 with the 32's
since my old tires were the 275R15 and Ive been running 30x9.5 it says to go 3.8 Gear Ratio. So probably that or the 4.56 with a posi/locker.
COLLIE RUNNER- what year was your buddies dime? Hard part about mine is its a 95. I dont have the chip and the K&N is made for 96 and newer. My K&N air filter is just the regular looking air filter from K&N that you clean and reuse.