Start at basics and think about what is actually happening. The fuel is igniting in the intake or an intake valve is open at the wrong time. Typically a backfire is going to be cause by timing. There should be nothing in the intake to ignite the fuel.
Could you have possibly swapped a plug wire? Did you rotate the distributor to far one direction when you did a tune up? (would have to be WAY off) This would cause a cylinder to fire when the intake valve is open. Does this happen randomly or all the time?
The only sensors I would consider would be ones that effect timing. I doubt that is the problem, but it is a possibility. I would start by pulling the valve cover and putting it at top dead center. Then visually look at the valves and make sure the ones that are supposed to be closed are actually closed. Rotate the crank slowly watching the valves to make sure they are moving smoothly. I do not remember if this is an overhead cam or not, but you should be able to see something not right.
If you can duplicate the backfire, start removing plugs one at a time till the back fire stops happening. This will at least give you the cylinder that is causing the problem.
Another idea would be to do a leak down test on the cylinders. I bet you will find a sticky valve.
Hope this helps. Don’t over think it, something is happening when it should not be happening. I would chase timing, either a mechanical cause or an electrical cause.
Not ruling it out, but I do not see how low fuel pressure could cause the problem. Easy check, so can't hurt. I just do not see it.
On a side note, I had a YJ doing horrible things. It was the ground wire on the back of the block. I do not remember it backfiring, but ran like crap.
