I would highly suggest 4.56 gearing. If the size of your stock 20's is 275/60-20 then that tire should be 33" tall. I have 4.56 gears on mine with 315/70-17 tires which are supposed to be 34.4" tires (measure 33" while on the truck) and my rpms at 70mph in OD are right at 2,200. The funny thing is, fuel efficiency has gone up since the gear change from the original 3.92's with these same tires for both city and highway driving. It hasn't been earth-shattering, but between 1 and 2 mpg average. I am VERY HAPPY with my decision to go with 4.56's, which was based in part on the suggestions and reviews by many others just like you are beginning to see. My biggest concern was protecting the transmission! I don't haul very often, but when I do, I won't have to worry so much.
To help you with your decision, go to
Accu Auto Parts - Installation and Tech Help Apps and click on "
Tech" located on the upper right corner of the red bar area. On the page it sends you, click
"Gear Ratio Calculators" located on the left side of the white area. This will get you to a very informative calculations page that will let you know what to expect. I have used this many times in the past and it is very accurate from what I have found.