1995 Ford Aerostar Driving funny

I am trying to find out what might be wrong with my van. It has started driving funny. When you drive it, it's like being in one of those vibrating chairs. It is worse at low speeds or when idling. It runs great around 35 mph. The rest of the time when you push on the gas it runs rough. I had the fuel filter and air filter replaced. I also added fuel injector cleaner to it. I thought maybe I had bad gas so when I ran out I refilled with mid grade and put in a fuel additive, but it is still doing it. Could it be my fuel pump or my spark plugs? I don't know which way to go next.