There are 3 different types of ignition systems which were used on 4.3s. The older ones used a distributor with a single coil that fed one terminal in the distributor cap; and the other wires on the cap fed the spark plugs in all the cylinders. Beginning in 2008, on 2 wheel drive vans; the 4.3 was switched over to a newer style ignition system; in which the distributor cap and single wire coil was eliminated and replaced with a long rectangular coil pack that had 6 terminals for 6 plug wires on it. This system is called "distributorless ignition." In this system; each cylinder's plug wire goes to one of the coil pack terminals. Here's a link to a photo of the distributorless coil pack in the newer style system: http://www.rockauto.com/catalog/moreinf
The system used from about 1996 to 2007 had a distributor cap and a 1 wire coil. This type of cap has horizontal towers with 3 towers on one side and 4 towers on the other side. The coil wire went to the distributor cap; along with each cylinder's plug wire. Here's a link to a photo of the distributor cap in that style system:
Before 1996; the 4.3 used a different type of distributor cap with vertical towers; The coil wire goes to the tower in the center of the cap, and the plug wires go to the ring of terminals on the outside of the cap: http://www.rockauto.com/catalog/moreinf
A coil pack is a coil with wires going directly to each cylinder, and no distributor cap. A coil with just a single wire is not called a coil pack; it is called a coil, and its single high voltage wire always goes to a distributor cap.
I hope this shows you why we need to know the model year of a vehicle when writing in with a question.
If your vehicle uses the 2008 and later coil pack; there is an
individual coil in that pack for each 2 cylinders; and one of those
coils can go bad while the others still work. A bad coil often does not
go completely dead; but it will misfire under conditions where a good coil does not misfire. But the fuel mixture strength is not the same on all cylinders: So if one coil in a coil pack is weak; it may misfire on one of the 2 cylinders it fires; while if the other of those 2 cylnders has a slightly richer fuel mixture; that cylinder may not misfire; or its misfiring may be less noticeable.
If your vehicle uses a distributor cap; the cap can develop a carbon track or crack which allows the current for one plug wire to leak to ground. This will create a misfire on one cylinder. A close inspection of the underside of the cap (in bright light and using a magnifying glass) can sometimes show the crack or carbon track; but sometimes the short will be in a spot where it is not visible to the eye. Some brands of caps are poorly designed, and are prone to such shorts. Other brands of caps are much more reliable. I prefer either Standard Motor products caps or Airtex/Wells Premium grade caps (which have an "A" suffix in the Airtex part number, or a "G" suffix in the Wells part number.)
Misfiring on one cylinder can also be caused by vacuum leaks; either in a vacuum line that connects to that cylinder's intake manifold runner, or a fault in the intake manifold gasket, or in a crack in that cylinder's intake port, or a burned valve in that cylinder. An EGR valve that sticks open can also cause this missing, as can an air leak at the PCV valve hose.
If there is a head gasket leak; it can also cause misfiring on one or two cylinders. Running a cyllinder compression test can show this; as any cylinder with compression pressure that is at least 15 psi lower than the other cylinders will cause this. But low compression can be caused by problems with valves, or pistons, or piston rings, or the head gasket.