Why did it have to be a 1999? That's the year they relocated the FPR (fuel pressure regulator) from the fuel rail to inside the fuel tank.

That gizmo on your fuel rail is a fuel injection pressure damper.

(if it failed you'd be running lean NOT rich.) (no black smoke).

Don't ignore that a rich condition can also be caused by NOT Enough air as well as TOO MUCH fuel.

Like a dirty air filter. Don't assume all is well. Open the air box and look at it. 

So, to answer your question, a fouled/"non sparking" spark plug won't cause black smoke.

It will however dump unburnt fuel into the exhaust through that Cat and that's not good.

That's not to say you shouldn't pull that #6 spark plug and take a look at it. 

You can tell a lot by examining a spark plug and there's plenty of charts online to help you.

So staying with your "Spark Issue", your Ranger uses a "Wasted Spark" ignition coil pack.

Your coil pack has 3 coils in it for the 6 cylinders.

2 cylinders fire at the same time. In the case of the #6 cylinder, it receives spark at the same time the #2 cylinder receives spark. 

Here we should look at the 4 stroke design keeping in mind that when your #6 piston is approaching Top Dead Center (TDC) on the compression stroke, your #2 cylinder is approaching TDC on the exhaust stroke.

So both spark plugs "fire" at the same time. The #6 cylinder is full of a compressed air/fuel mixture so it ignites.

Your #2 cylinder is exhausting "spent" combustion gasses. It can't ignite. ( "Wasted Spark") 

So now let's look at your coil pack/engine cylinders.

We can see that cylinders 1 & 5, 2 & 6, and 3 & 4, share the same coil in the coil pack.

So if it were a coil pack issue we'd expect to see that same cylinder #6 misfire on cylinder #2.

But what we expect doesn't always happen. "Spark" isn't always "Good enough spark". You should buy an HEI ignition spark tester to test the "quality" of the ignition spark to that #6 cylinder (they cost under $10 bucks) to rule out that the P0306 code is being thrown because of weak spark.

So I've "ladled" my point on ignition long enough but one more thing about that misfire. Don't ignore a "shorting out" Spark Plug wire. (good luck finding just 1 in the length you need. When I had one arcing on a valve cover stud I had to buy the set).

So the reason I went on so long about an ignition issue is because they're easier to test than a fuel issue and that's what seems to be happening here.

You're running Rich. Is just 1 cylinder bank responsible for it or are both?

It would be useful to observe the Fuel Trims on both cylinder banks.

You can get an inexpensive OBD2 scanner that can do that for under $40 on Ebay or Amazon.

If only 1 cylinder bank is causing the rich condition we can rule out a lot of stuff.

Like, MAF, IAT, ECT sensors, fuel pressure, or air intake issues.

Here we'd suspect something more isolated like maybe an injector that's leaking a little.

(which we could easily diagnose with a fuel pressure gauge ($20) observing the fuel pressure "leak down" rate)

check out the misfire troubleshooting in our FAQ

Follow-up questions...If the cylinder is misfiring because of lack of spark, will black smoke come out of the tailpipe? 

Can fuel injectors dies slowly and intermittently? The injectors are original. 

Can sparkplugs and wires dies slowly and intermittently? The plugs and wires are OEM, 100,000 mile intervals and only have 19,000 miles.  

I got the code on the way to work. I live in the SW were there is a heatwave. I leave around 12:50 PM. I return home around midnight. I did get the engine shakes, and the CEL 0306 (If I remember correctly). When I started the car for the drive home, it was smother. It later started shaking. I did make it without the CEL, but the 0306 was pending. I started the truck about a half hour later. I have it some good revs. I then checked codes. I had a P0174 System too lean bank 2. before I swap pugs and get new wires, does that sound like a bad injector? 

 This is cylinder 6's plug. Can can a plug be this clean with an intermittent ignition problem? The problems come after the engine heats and runs for a while. I also got a stored 0174, system running too lean bank 2. Does it now look like a fuel issue?

This problem is beyond my abilities to fix. I know a little about engine, but this is beyond me. The sparkpug being not fouled and the bank 2 running too lean tells me that it's a fuel delivery issue. I'm guessing #6 injector is weak, but I don't know why the truck starts out running well. Actually today it didn't take long to get a bank 2 running too lean. I'll find out more Tuesday.