No not a common issue.
4.0l OHV engines eat spark plugs, and they do need the MAF sensor cleaned more than other engines.
Gotta run a big gap on the spark plugs, 0.054

What did the spark plugs look like when you pulled them out?
10mpg should look dark brown rich running.

I would hook up a vacuum gauge to intake and do some testing
Good article here on that: Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine

No matter what "they" add to the outside of a gas engine, it is still a self powered air pump, so testing vacuum pressure can tell you ALOT about what going on inside the running engine.
If you don't have a vacuum gauge, it will be the best $20 you ever spend, it works on the most high tech gasoline engines made, or a lawnmower engine, lol.

I assume no engine codes?

The older computers have 3 capacitors that can leak, which can cause issues until they fail completely, you do need to pull out the computer and open it up to look at the 3 blue capacitors for signs of that.
This is what a clean one looks like inside: http://www.auto-diagnostics.info/ford_eec_iv
Under $10 to repair, if you can solder

Lack of power causes the lower MPG overall
But computer running engine rich can cause the lack of power, lol
Spark plugs ends tells you which is happening

Lower compression causes loss of power, 4.0l runs 9.0:1 ratio, so at sea level to 2,000ft you would expect 150-160psi in each cylinder
Compression level is set by valve timing and piston rings, if compression is low then add a teaspoon of oil and retest, if compression comes up to "normal" levels then a ring problem, if it just comes up a little then valve timing issue or a valve in that one cylinder

Slow flow out exhaust pipes cause loss of power, but on this you would usually hit an RPM limit, where engine couldn't go higher
Same with low flow from air cleaner, higher RPM issue

Spark timing is very important for power
1990-1994 4.0l used EDIS-6 module for spark, it controls RPM spark advance, but it needs a SPOUT signal from computer for "vacuum advance" info.
Distributors had weights in side for RPM spark advance, but need a vacuum advance for best power when driver "stepped on the gas", without this engine would be sluggish under load and have limited power
So computer may not be feeding EDIS module good info for spark advance

 

 

Light brown means engine is not running rich so you are down to compression and spark timing.


Computer controlled engines have monitored sensors so you will get a CEL(check engine light) if a sensor has a problem.
Which makes these engines very easy to diagnose, not the pain in the butt we had with carbs and points, lol

Run the vacuum tests and see what it tells you

 

I just can't help myself Ron...
Carbs and points are infinitely more simple then an electronically controlled fuel injection engine ever was.
I've had British and American fuel pumps go for 20 plus years or more with no maintenance, you can't say that about a fuel injected vehicle.
Ignition coils that are 20 and even 50 years old that just keep going.
(although I have replaced them)

Even had one guy one guy who couldn't remember when he changed his points, must have been in there for years.
They were worn to a frazzle, but still worked, although to be fair, it was a duel point system.


The problem with todays technology is that when things go wrong, it can be very difficult to find the problem because such little things can affect the system and finding the problem can be like finding a needle in a hay stack. Even when one knows how to read the codes and the symptoms, it doesn't always point to what the problem is, I find myself having to be smarter then the system and think out side the box.

Stupid things like a slight bad connection, faulty condensers on a circuit board, a slight bit of dirt on the sensors.
Everything requires maintenance, it's much easier to diagnose and old system then a new one when things go wrong.

I've never touched the carbs in my both my old Mazda B2200's.

Had a lady with an expensive Lexus SUV, ran fine, but the check engine light was on.
The dealer, after trying to figure out what was wrong said, "it must be the PCM" and wanted to replace it and a great cost to her _ out of warranty.
The part that bugs me, is that if it didn't solve the problem, she couldn't return the PCM.
I guess in that respect she had a bad mechanic, but it was the factory trained mechanic at the dealer...


This sort of thing would never happen with old points and carbs.