ok think of it this way. the rear end gear determines the ratio of how many times the tires spin relative to how many times the driveshaft spins. so by increasing that ratio (going from a 2.73 to a 4.11) you are going to limit your top speed, but decrease the length of each gear (ie youre in your motor's powerband more) if you decrease the gear ratio, you will increase your top speed (which is relative to gas mileage because you wont be riding at such a high rpm on the highway) but it will take longer to reach your peak power and get through that gear. one way to get around this is to use an overdrive transmission. if you switched to an overdrive transmission (4l60e, th700r4) you would have an additional gear that is meant for cruising.

if i was you, id swap trannies if your intent is to drive the truck everyday. if your intent is to have a wild track truck, leave the 3spd in there and spend money on a good stall.