Welcome to the forum
Rear leaf spring hangers on pick ups rust out faster than other stuff, keep an eye on those.
Best and easiest is to remove the bed, 6 bolts, filler tube and tail light wires, then just lift it off.
Change all 4 hangers at that time, they are not expensive.
Also removing the bed or sliding it back is best way to change/service fuel pump.
Check for throttle cable stretching, not uncommon on Rangers
Press gas pedal to the floor and put something on it to hold it down, weight or stick against the seat.
Go to the engine end of cable and see if you can manually open the throttle more.
If so Google: Ranger throttle cable mod
Easy fix, couple of zap straps/cable ties on throttle cable end in the cab.
That could be why "kick down" for trans isn't working like it should, kick down is initiated by throttle position, TPS(throttle position sensor) is how computer decides if it should drop down a gear.
2002 also uses OSS(output shaft speed sensor) on transfer case output shaft for accurate speed reporting.
This is set for tire size and axle ratio.
And computer uses this speed signal for shifting trans gears.
If tire size is changed(or axle ratio) then speed is no longer accurate so shifts wouldn't be either.
Is your speedometer correct?
The big deal for 2002 4.0l SOHC engine is the time chain set, 1997 to 2004 4.0l SOHC engines had a bad design for the timing chain guides(cassettes), they deteriorated and chains started to rattle and it could throw timing off or worse, 4.0l SOHC is an interference engine
, so valves can hit the pistons if timing gets too far off.
Because there is a rear timing chain the engine must be pulled out to install new cassettes and chains.
The newer design and parts are bullet proof so once changed all is well.
This isn't a 100% failure rate, following normal oil change routines seems to prevent issues, but isn't a guaranty.
The PCV valve hoses have elbows that can crack after 10 years or so, they crack on the underside so "look fine", you will get CEL(check engine light) and Lean codes, which usually indicate a vacuum leak, PVC hoses are first place to look.
Good tech article here: PCV Elbow - Fix for a very common CEL!
Shift transfer case in to 4hi and 4low once a month, this will keep shift motor working, you don't have to drive it just shift it.
If the shift motor sits for many months without moving the brushes can get stuck and no longer make contact with rotor, so when you need 4WD you don't have it, tapping on the shift motor can often restore function, but it would need to be pulled off and cleaned or replaced if that happened.
Rangers overall are one of the better trucks made, it is a Ford truck so that is just expected, 300+k miles on chassis is normal, my '94 is still going strong at 350k.