1995 GMC Jimmy Runs rough except on flat road

My truck seems to run fairly smooth while driving a constant speed, but if I accelerate or begin slowing down, the engine begins to run rough. Also, as my transmission shifts through gears, the engine roughness changes, almost like it runs rough only at certain load/rpm/gear combinations. I hear that if it runs rough ALL of the time, it's probably a timing issue. In this case, would my problem have more to do with spark plugs/wires/distributors than timing? Or something else? Thanks.