[Benzman]

Yes, the driveshaft can be out of balance. I would start with balancing all 4 tires. Then, look at how much weight was required to balance each tire. If one tire has a large amount of weight on one side, then this tire is probably out of round and even though it's balanced it's like riding on an oval tire.

Also, before you take the car in to get balanced, wash all of the mud / dirt / grime off of the rims. For some reason, the tire people don't think this is important. But, think about it. You take your rims in with mud on them, they add weights to balance out the tire. The mud eventually gets washed off and the tire is no longer in balance.

Could be lots of other things, so try this first and write back.