1998 XJ Rough Idle and Mild Backfire

I have read dozens of threads trying to glean some info but at this point noting seems to fit exactly so here is the situation. 1998 Jeep XJ I6 running fine. I decided to change out the air filter, spark plugs, wires, distributor cap, and rotor with the intent of getting a little better than 17 MPG and I had no idea when it was last done. The plugs I removed were Autolite Copper Core 985's and were gapped to about 0.020 oddly enough. I replaced them with the exact same plug gapped to 0.035 per factory specs. The plug wires, distributor cap, and rotor looked good but I replaced them just because I already had new parts. So after doing all that I fired it back up.

Now. Rough idle with a mild backfire occasionally (every 10 - 15 seconds). Check engine light would come on likely indicating air/fuel mixture not right. I verified all the plug wires, connectors, and everything in the area I was working and all look ok. Still same issue. So I changed back to all the old parts and still the same issue. I even confirmed spark on every cylinder and the engine definitely runs worse when each plug is pulled. The real problem is, since it is not throwing any codes, I am at a loss what to do next. I have driven it a few miles and it is a dog, basically in limp mode. What I can't figure out is why this happened after the tune-up? And why it continues when everything has been put back to its original state. Which leads me to believe it is a sensor but which one? I can't justify justify replacing every sensor because it might be the problem. All I have is a basic OBD2 scanner which really doesn't tell me much other than trouble codes. And the check engine light does not come on anymore, even when driving it on the road. I did have to pull the battery for a few hours, could the computer have reset and allow this issue to become apparent where it was masking it previously?

As I said, I want to diagnose the issue, not just start replacing sensors hoping the problem is fixed. I am just not an engine guy and the cost of gas driving my TJ is killing me... TIA.

Before you said you put the old parts back on you said the check engine light came on. Even if it's off it would have stored a code. Did you pull the actual code before you pulled the battery? If you pulled the battery for a few hours the code may be gone now and it "could" be re-learning. I read on here it takes approximately 50 starts to gather enough data while it's learning.

Backfiring is ignition of the remaining HC in the exhaust gases due to introduction of oxygen through an exhaust leak.

Find and repair the exhaust leak.

No codes stored, I did try to pull them...

Resolved. Plug wire order was wrong,.. 1 and 2 reversed.