Short Term Fuel trim with -1 or lower is rich, +1 or higher is lean, these numbers represent Fuel injector Pulse Width, how long an injector is open.
-10 to +10 is in the normal range, computer will usually turn on CEL(check engine light) when short term fuel trim gets to -20 or +20 area
If you have a - or + you need to have a 0, that 0 comes from MAF(mass air flow) sensor and the computer's 14:1 air:fuel ratio calculation based on the MAF sensors air volume data.
So if fuel trim is -20 then computers initial 14:1 calculation is adding too much fuel to the available air, there is less air than reported, so computer doesn't open injector for as long(leaking injector possible)
If +20 then there is more air than reported(vacuum leak possible) so computer is opening injectors longer, adding more fuel
O2 sensors read Oxygen levels not fuel levels, high oxygen = lean exhaust, low oxygen = rich exhaust.
Unless engine is pinging/knocking(lean) or blowing smoke(rich) it isn't actually running Lean or Rich, computer is adding more fuel or reducing the fuel so engine runs normally, a CEL for either Rich or Lean is just letting the driver know something is wrong, the calculations are not adding up.
A misfire dumps oxygen into the exhaust, it didn't get burned up, which means O2 sees lean(high oxygen levels) and computer adds more fuel.
So a misfire can cause Lean fuel trim.
So I would address the misfire first, 4.0l engines are known for being hard on spark plugs, over the years I have had a spark plug just fail while driving, motorcraft tend to last longer but they can still just fail one day.
Runs fine then it doesn't, just like that.
One spark plug died and looked fine, tested it on an old coil I have and it was deader than a door nail, oddest thing, not that old either.
So if you have a steady miss, i.e. same cylinder, then I would check each spark plug.
Also people often check the coil pack and sometimes goof up the firing order on these.
3 4
2 6
1 5
Front
That 5 6 4 gets me every time
The fuel trim numbers may indicate low fuel pressure but long shot, would need to see the actual numbers at idle and then driving at 40mph
How many miles on the O2 sensors?
The front O2s are rated for 100k miles, if they are well past that then replace them, they may not be the problem but you will get better MPG for the next 100k miles so worth it just in general maintenance.