Engine vibrating

Hey guys!

First, thanks for your time.

So my van has been driving very well since maybe three to four months ago. Slowly, I could start to feel vibration when I was going up a hill. The van started to check a little. And more those months passed, more my van shakes when it needs torque. But it is not consistent. Sometimes, it starts to check quite a lot, sometimes it is smooth.

It started to check on the highway when I am close to eighty mph I release the gas pedal and it stops, press again it could be still shaking or not.

It shakes pretty much when the van needs power.

I do not know when was the last time the timing belt was changed. I suspect it is from the engine timing. Should I start by changing the spark plugs and see?