A bent axle shaft is highly unlikely. A bad wheel bearing could make the noise that you're hearing but wouldn't create any vibration. I'd be looking at your driveshaft u-joints as the first and most likely culprit. Do you have any pulsation in the brake pedal when stopping? A seriously warped brake rotor could create vibrations that you'd be able to feel while driving as well as when braking.

If you're going to check it while it's up in the air and running, make doubly sure that you have it securely supported with your jackstands under the frame - I would leave the front wheels on the ground and chock them securely. Take the rear wheels off and have someone run it at the offending speed while you look and feel what it's doing without getting underneath it.

If you don't feel the vibration while running it with the wheels off the vehicle, you've probably got an out-of-round tire. Tire shops would love to have you think that when it's been balanced, its going to run smoothly. That's not the case, you could balance a square tire if you had to but it doesn't mean that it'll ride smoothly. You might need to have the tire road-force balanced, which measures the force that the tire puts against a roller and can detect an out-of-round tire.