Isn't the tranny in your 1997 the E4OD, meaning it is a 4-speed (3+ Overdrive)? I ask because the newer 6-speeds include OD in the "6", so yours isn't quite as far behind as you may think.
When you are cruising the highways, the truck will generally be in its top gear (overdrive) and that will be a pretty much identical gear ratio whether the tranny is 3-speed, 4-speed or 6-speed. The exact gear ratio in the top gear varies a bit with the rear axle gearing, tires size and engine torque/hp, but all other things being equal the final drive ratio will be the same. The advantage of more gears is better power application between standing starts and cruise speed, e.g. acceleration and hill climbing. You gain a little mpg around town and on long grades because a more optimal forward gear will be auto-selected, but for most owners that is not a large factor in overall mileage.
Most 6-speeds (or the newer 8 or 10 speeds) have multiple overdrive gears, so have more choices that still produce better mpg. Your E4OD kicks out of overdrive at the slightest load and goes to 3rd. On any sort of grade, it probably hits 2nd gear, which is a substantially different ratio and really hurts the mpg. A 6-speed can hang in and use 5th or 4th or 3rd in that same range, and only resorts to 2nd gear when the going gets rally tough. That can help mpg quite a bit in those circumstances. If you do a lot of driving in rolling hills, you will notice the difference in both economy and perfromance.