That could be acouple things, depending on how severe the vibration is. Tires are a good place to start as it's easy and inexpensive to get them computer balanced and a reputable tire dealer. Often you can get a deal on lifetime balancing and rotation.

If it is indeed related to the propeller shaft the need for replacement is extremely rare unless the shaft itself has been damaged. If your tires check out okay, then the rear shaft would be the best place to start troubleshooting. You want to check your universal joints for play and make sure they're properly lubricated. There's also grease zerk at the spline coupling that needs to regularly lubricated as well. Also, driveshafts can be balanced. So if you simply can't find an obvious problem go to a reputable driveshaft shop and look into getting the shaft balanced which is simple and far less expensive than replacement.