Here's a chart from Grimm Jeeper with the 700R4, NP208 (at 1:1), 35" tires, and on the left is 4.10 gears and on the right is 4.88. Should help you see what to expect, looks like closer to 2300 rpm at 70 mph. If your 4.10 rpms match the chart when it downshifts, I would go ahead and pull the trigger on 4.88s. I would still only expect 10-14 mpg, 10 towing, 14 under the best of conditions. Keep in mind driving these trucks on the highway is like trying to carry a sheet of plywood in a windstorm, definitely hits the mpg hard. Hope for a tailwind.