First off, DITTO, on everything RonD posted.
$0.02 - I have a well worn 2.8L that idles about 1500-1800 rpms, base 14+ degree timing, running duraspark ii, 160 or 180 thermostat (I forget), no fan shroud (yet), and I have no issues with dieseling. Since you did not have problems, but, now you do, I would look at what changed. Most likely a vacuum leak or carb base leak. I would not rule out the PCV. If you have vacuum caps, get rid of them, and use a hose with a bolt in the end and a hose clamp around the hose on the bolt end.
I would say it is about impossible to get a good quality vacuum cap anymore because now they all come from China and that is the only kind you will find at box stores. They get holes, they crack, they fall off, they do not seal, they are a headache waiting to happen. Rice racer silicon or rubber, they are all inferior to what was a simple reliable item 20 years ago.
If your 2.8L is still pretty much stock, you might have a problem with the charcoal vacuum system and the canister is loaded down with gas and the carb is not pulling fumes out or dumping them into the canister when the vehicle is shut off. Very easy to check, just remove it and see if it is loaded with gas vs. just fumes. Located on the driver's side next to the radiator.
Sometimes adding a 1" Phenolic Throttle Body Spacer can cure the dieseling without doing anything else. Though that will not fix the problem, just mask it. I run red pump QT (no alcohol) in my 2.8L and it makes a huge difference in drive-ability. My neighbor noticed improved performance once I convinced him to use it for his SHTF supply, which he then used it in his 2017 Expo with the twin turbo 3.5L a few months later.