If you suspect a u-joint is going bad, don't waste your time changing just one, do them both. Sometimes a bad u-joint will not be detectable by the usual method of just trying to turn it to see if there is any play in it. If there are alot of miles on the vehicle, or you carry heavy weight, or tow with it, it's probably time anyway. As far as changine the gears for better mileage, I'll leave that up to someone else.