Better not buy the Ebay kit. Looking like Genuine brand gearset with the ring gear bolts included. I am going to guess you have 4.56 gears. 4.88 will help. That's what came with you truck IF it was stock with 31 x 1050 tires. (gear code G144, V6, Auto) 5.29 will probably be better for you. You can purchase used 4.88 diffs from car-part.com for a more reasonable price though, then just plug-n-play the entire diff. Your call. Also, check codes to see if your O2 sensor is defective. This will also cause bad fuel mileage.

85 4Runner - With NEW Marlin 4.7 Gears!! It's a whole new beast!