If you have 4 wheel drive you must have low range in your transfer case. I would imagine you also have a 4 speed trans. Your stock 3.91 gears are optimum for a combination of fuel mileage , highway cruising, and low end pull. If you go to lower gears you will lose any use for 1st gear in low range. Any higher will kill any performance on the highway.
If you have an automatic you might consider lower gears and a higher stall convertor. Remember that lower gears can be very hard on drivetrain parts on 4 wheel drives.
If you decide to go the posi/locker route dont let anyone convince you to do the front diff as well.
Good tires beat lockers every time.
If you want to run 5:38s get a taller tire for ground clearance. (33 or 36) but remeber it takes an exponentially greater amount of HP to turn larger tires.
My work truck makes just shy of 300 rwhp and I run 31" tires (265 75 16E), 3.71 gears and an Allison convertor. Chained up on all 4 in low range it pulls a loaded 12000lb trailer through 18" of wet snow or mud.
