My van shakes and shimmys

Hello,

You might have a vacuum leak. Many times, a vacuum leak would be more noticeable a low engine speed but less noticeable as the vehicle drives and has higher engine speed.
It could be due to cracked hoses, it can also be due to leaking intake gasket.
Inspect for any loose or damaged hose. The intake gasket leak can be tested by spraying carb cleaner at the base of the manifold with the engine running. If there is a leak the engine will react to it and idle high. Then it will go back down.

Take a look at this guide, it covers the typical issues and how to check them:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Run through this and let me know what you find.
Thank you.