Your '93 4.0l will use an EDIS-6 module for spark timing, usually located on the rad support, can be the front side like my '94 or back side usually by battery tray.
These are pretty bullet proof and can run the spark system without computer input, but they are hooked to the computer so computer can set correct injector timing from spark timing.
Computer also gives EDIS module spark advance info, i.e. you step on the gas, TPS(throttle position sensor) voltage goes up, computer tells EDIS to advance spark so there is less lag, like an old vacuum spark advance but less delay.
Spark timing can of course cause backfires.
But since Unplugging O2 sensors would not change anything on this I can't see spark system being part of the problem.
BUT.........Ford EDIS is a "waste spark" system, meaning each cylinder's spark plug sparks twice on each 4 stroke cycle, once on power stroke and one on exhaust stroke, this can come into play for backfires.
There are two kinds of backfires
From intake and from exhaust
Usually intake backfire is a sign of Lean mix, lean mix causes cylinder to heat up, then when lean mix comes in via open intake valve the it self-ignites in overheated cylinder.
Backfire is heard.
Exhaust back fire is usually a Rich mix, on power stroke not all the fuel is burned but all the oxygen is, when exhaust valve opens unburn fuel gets oxygen and can self ignite that's in the old days, Ford's waste spark helps that along with the second spark on exhaust stroke.
Since disconnecting O2 sensors seems to lessen the backfires I would assume this is an air/fuel mix issue, not spark timing.
So, with O2s hooked up, does the engine ping/knock while under load, thats a sign of Lean mix?
Does the engine exhaust smell rich or are the Cats getting Very Hot, thats a sign of rich mix?
Leave O2 sensors connected and unplug MAF sensor, MAF sensor is the main sensor the computer uses to set the 14:1 air:fuel ratio needed.
Also on the computer side, Calif. computer needed Cam Position Sensor and EGR Valve, if your Truck is from Calif. then you need a computer for those, if it is not then you don't.
And they can't be swapped around