Why is my car vibrating past 50 mph?

About a week ago, I had the tires rotated on my 1998 Ford Contour. Before the rotation, the car had a poor alignment and pulled slightly to the right, especially at higher speeds. However, there was no vibration at speeds of 45 mph and over. That is what I'm getting now after getting the tires rotated.
When I get to highway speeds of about 45-65 mph, I get the vibration. However, when I get past about 70 or so, the vibration lessens. I do have one tire that is slightly more worn than the other three, but it's on the rear (car is front-wheel drive). The tires in front were purchased together and are evenly worn. I'm also certain the tire rotation itself was done correctly.

Do I just need to get my tires balanced?

Note: I had an alignment done because I thought that may have been the problem, but it wasn't. Car just doesn't pull to the right like it used to.

You said you rotated the tires then it started to vibrate. Its possible that your front tires before the rotation is already balanced but you reasr are not. When you had your rear tires moved to the front then you have an imbalance front end tires. Have the two front tires balanced.

Those are symptoms of out of balance

Tires could be out of balance or when they installed the tires onto the vehicle the may have not tightend the lug nuts to the proper torque. This is not noticeable at low speeds but significant at higher speed. First jack the car of the ground and shake each tire if there's any play in the tire then they need
to be tighened.

Get a new or part worn tire on the one that is low not just for the vibration but because this can carry a 100£ fine and 3 points on your license for each bald tire. Then get both front tires balanced and have your tracking done!

Tire is out of balance. You need to get the tires balanced and rotated by a shop so they can add the correct weights.

At least one tire is out of balance.

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