Check engine light is on, runs rough when shifting gears

94 Nissan pickup, check engine light is on and it seems to be running rough when accelerating and when shifting gears, especially when going from 4th to 5th gear. Runs smoother when at speed, but loses power when going uphill and runs rough again.

Changed plugs, plug cables, fuel and air filter. O2 sensor bad maybe? Vac problem? Not sure where to go next with this.