Duratech engines vs. Lima
I had the old Lima 2.3 in my '93 Ranger - which I gave to my son and it still runs like the proverbial "top". Having had both, I find the Duratech to be a smoother, higher reving, and definetly a more powerfull engine in comparison. I think they went to the Duratech design because it's aluminum - therefore lighter weight. We learned in Dynamics (many years ago!) that one way to dampen vibration is to increase the system mass - not decrease it.
What you are experiencing sounds like it "came with the truck" from the beginning, whereas what I hear/feel just started (- come to think of it after I put on my snow-tires and installed my weight in the bed for winter - hmmm.) Also, I have a stick so that there is less "load" on my engine at an idle because I am not spinning a slipping torque coverter.
I suppose they checked your @#$! IAC valve for proper operation. That thing is just a vacuum leak waiting to happen - thanks EPA!! My older son had an Accura that had IAC trouble and I bet it ran rougher than your Ranger and your Dad's K-Car put together !! The dealer told him he must have a cracked-head, blown head gasket, intake leak, on and on ( you know, out of warranty!). I replaced the IAC and it ran fine -then he sold it because he learned that a Honda IAC valve costs almost as much as a Duratech engine!.
Anyhow, please repost anything you find.