Check these things!
There are a couple of things you might want to check before worrying about the balance. First, check the nylon bushings that center the driveshaft. These are located in the ends of the driveshaft, and are replaceable. They center the driveshaft, and if they are worn, will cause the shaft to be out of balance. Sometimes they will cause severe vibration starting off from a stop, but then as the speed increases, the vibration goes away. Secondly, check the center bearing and U-joint for wear. The U-joint in the center will also cause the driveshaft to go out of balance if worn. The driveshaft is very closely balanced at the factory, so almost all problems are due to wear or damage. Good luck!