Great news! I fixed the van today. So simple, and I never would have guessed it was something so minor.
So after further research I decided to check the MAF (Mass Airflow) sensor as a dirty or faulty MAF sensor can throw error codes and cause symptoms like what I was experiencing:Ford Mass Air Flow Sensors
So I disconnect the MAF, start the engine, no change, still running crappy. So I guess that rules out the MAF. Next, I spray ignition fluid on vacuum lines while engine is running and check for vacuum leaks from the hood. Can't detect nothing. So I turn it off and remove the interior engine cover and search for vacuum leaks. Everything looks fine. So I start it again and continue searching for vacuum leaks on the back of the motor thru the open interior engine compartment cover. A blue spark catches my eye. One of my spark wires is laying on the exhaust manifold and is arcing into the iron. Impossible to see from the front of the motor. I didn't think it would fix the problem, but I knew I needed to move that wire. So I shut it off, move the wire off the exhaust manifold, start it up again, and it runs perfectly! You could have knocked me over with a feather. Couldn't believe it. Cleared the error codes, went for a good 15 minute test drive, it ran flawlessly. Checked the codes again after I got back and the 172 code (No HEGO switching detected, indicates lean [right]) was no longer there. Awesome! Only trouble code present was 452 (Insufficient input from VSS), which could explain why my overdrive doesn't work (VSS=Vehicle Speed Sensor). The overdrive hasn't worked since shortly after I bought the van, (speedometer doesn't work either), but I just drive slow on the highway. Next, I'm going to check the VSS. From what I've read in the Haynes manual, it should be fairly easy to replace. That would be sweet if fixing the overdrive turns out to be simple as well.
I feel like I dodged a bullet here, was thinking it was going to be a major repair (even worried I had spun a rod or crank bearing), so lucky it was just an arcing plug wire. Guess the misfiring cylinder really threw the engine electronics out of whack, hence the 172 error code and lean spark plugs. I had a feeling it wasn't firing on all cylinders.
Took me 4 days to figure it out, but I sure learned a lot about how to troubleshoot computerized engine systems, which is all new to me.
Thanks for all the great help Bert.