If you have no vibrations at any speed now with the aluminum shaft, you "shouldn't" have any after new gears if you take extra care to line it back up with the output shaft and the pinion flange just like you have it now.
About the fit issue, some cars DO actually have a problem with the fit because of production tolerances. The reason Ford doesn't list a driveshaft compatible with the 99-01 is due to the fact that these IRS cars are extremely sensitive to imbalances in the drivetrain - there's a real problem with vibrations. If you are unlucky and the aluminum shaft is a bad match for your flange runout tolerances, it will vibrate. Ford doesn't want to take a chance of that happening, so therefore they don't want you to replace the driveshafts with aftermarket ones.