Vibration at 30 MPH definitely sounds like driveline (at that speed, tires are spinning very fast, so balance/roundness issues feel like a shimmy at slower speeds).
Double check the center driveline carrier bearing. Very common for those to wear out. Even with low miles age can make the rubber crack. Crawl underneath and give the driveline a good firm shake. If it moves, you need a new carrier.
And as mentioned.... check the U-joints too for play, and make sure they are lubed. Dry u-joints also cause vibration. If yours still has the factory "sealed" U-joints, they are probably due for replacement, at which time you can upgrade to grease-able ones.