Vibration at all speeds till 65 mph

My 2003 Ford Windstar LX van has 90000 miles on it. It has a 3.8 l engine. For the past two weeks, it started shuddering and shaking at low speeds till 45-50 mph. The steering wheel also vibrated along with the van. Now this happens till 65-75 kts. The speed is not fixed. The vibration is felt most severely at low speeds of 15-20 mph on the steering wheel. At high speeds, this shuddeirng of steering wheel reduces significantly.

The tires need replacement. One of the tires was not good and I had replaced it with a similar used tire. I am not sure if the shaking and vibration is only because of alignment/balancing/ worn out tires or there is another problem? :D :D