Engine running rough with check engine flashing

I bought this truck new in 2002, and have kept it well maintained. I backed it out of the garage today to go to the store, and when I started up the road it is running rough and shaking, and the check engine is on and flashing. I immediately turned around and went back to the house. When I crank the truck it still runs rough and shaking with the check engine flashing . I plan to take it to a garage tomorrow and have them plug it in to see what codes come up, thanks.