I would have the driveshaft re-balanced, or do it myself with washers, as stated in another post.
Think about it. With the factory gears the shaft was spinning like what, maybe 2500 RPM at 65MPH? with higher gears the shaft will spin faster. From the factory the shaft is balanced only to a certain speed, because when they balance it they only expect it to turn so many RPM's with factory gears.
When you change gears you change the shaft revolutions, and the shaft isn't properly balanced to the RPM's you are now spinning it with the new gears.
The perfered way would be to remove the shaft and have a reputable shop balance it, but i have read many posts here where guys are using washers under the bolt heads to balance it.
just my .02