Ram misfireing

My father in law has a 1996 Dodge ram 4x4 auto with a 318. It is running rough as if it was running on 4 out of the 8 cylinders and when he puts it in OD it acts like it is running on 7 out of the 8 cylindes. the truck does not do this all the time but most of the time he changed the cap and rotor but it did not help. any help on this problem will be great.