With the tires wearing on the inside, you have a toe out condition. Toe needs to be set.

The vast majority of vibrations are tire/wheel related. You need to set the toe at a minimum before or at the same time as tire replacement. When replacing the tires, make SURE that you get a dynamic balance with weights on both the inside AND outside lip of the wheels. If weights on the outside look bad, so be it. Don't let the tire store kids talk you into balancing the inside only. This is called a static balance, and there's not a car on the road that reacts worse to a static balance than a 123 MB.

The nut on the driveshaft simply pinches the pieces together. It should be snug, but even if it's not, it should not cause a vibration.

See to your tires, balance and alignment since you need tires anyway. In all likelihood the vibrations will go away.

My $0.02,