1998 Plymouth Voyager my van

More than likely this is a tire issue or excessive runout on a wheel or hub.

So, we need to start with finding out where you are feeling the vibration. Is it in the seat or the steering wheel? If it is in the seat then more than likely it is the rear wheels, tires, hubs. If it is in the steering wheel, then that would be the front.

Here is a guide that will help with tire wear issues:

https://www.2carpros.com/articles/how-car-tires-work

Also, below is a chart that will help as well. Please inspect the tires and we can go from there.

Thanks