What you have described, low power on hills and limited RPMs, really doesn't read like a sensor issue.
About the only sensor that comes close is the MAP sensor as mentioned above.
This sensor is used by the computer to calculate air flow into the engine, so computer can open the injectors the correct amount of time to get the 14:1 air/fuel mix.
BUT...........there would be stumbling and bucking complaints as well as low power complaints, if MAP sensor was failing.
A vacuum leak in the MAP hose is a different matter, check it.
'91 2.9l will also have a TFI spark system, these have 100% failure rate at this age, 24 years old, so some parts have already been replaced or will need to be.
BUT..........there would be other symptoms if spark or spark timing was at fault.
Good read here on TFI system:
http://www.therangerstation.com/tech_library/TFI_Diagnostic.shtml
And just for a statistic, over 60% of DIY sensor replacement was unneeded, sensors were good.
25% of professional mechanic replaced sensors were still good.
Here is a list of engine sensors on a '91 2.9l
TPS(throttle position sensor), tells computer where throttle plate(gas pedal) is, this allows faster acceleration response because of the lag time between "foot on the gas" and MAP sensor showing vacuum drop.
It has 3 wires:
Power 5volts
Signal back to computer, .8volt closed--4.6volts wide open
Ground
Very easy to test this sensor with $15 volt meter.
If this sensor only goes to 4volts then you will never have full throttle so less power.
Common issue on the Ranger is for the throttle cable to stretch a bit, so you don't ever get full throttle, easy to fix this issue, read here:
http://www.therangerstation.com/Magazine/Summer06/throttlecablemod.htm
Yes, that easy, put a zap strap on cable end
IAT(intake air temp) sensor, colder air is denser than warmer air, so colder air needs slightly more fuel to be added to get the 14:1 air/fuel ratio and full power.
These hardly ever fail and your complaint would be poor MPG more than performance issue.
ECT(engine coolant temp) sensor, this is a TWO WIRE sensor only used by the computer(there is a ONE WIRE sender used by the dash temp gauge).
ECT tells computer when engine is cold and warmed up, when engine is cold computer runs it richer with a higher idle, Choke Mode, as ECT warms up idle will starts going down and computer will lean out fuel mix.
Complaint if this sensor fails is that engine runs better cold than warm or better warm than cold, but ECT sensor rarely fails and isn't the only reason for engine to run better cold or warm.
O2 sensor, this reads the OXYGEN in the exhaust, not the fuel
O2 sensor lasts from 100-200,000 miles, as O2 sensor starts to fail you will get a CEL with a code about O2 sensor "switching slowly".
O2 sensor tells computer oxygen content in exhaust, too much oxygen means misfire or not enough fuel was added so most of the oxygen wasn't burned, Lean.
Not enough oxygen means too much fuel was added so no oxygen remained, Rich
Look at tail pipe and listen to the engine.
Black smoke at tail pipe mean too rich, and VERY hot Cat converter
Pinging/knocking in engine means too lean
If you have an EGR system you will also have an EGR sensor that tells computer if EGR valve is open and by how much.
Failing EGR system will cause rough idle and loss of power at all RPMs, also pinging if EGR is not working at all.
That is it for engine sensors, on the '91 as said above you do have the TFI spark system, it has a sensor/pickup that tells computer the RPM and when to open injectors.
On newer engines the TFI sensor was replaced with CPS(cam position sensor) and CKP(crank position) sensor.
You then have engine Controls
IAC(idle air control) Valve, this is a computer controlled valve used to set idle, it allows more air(higher idle) or less air(lower idle) to pass by the throttle plate.
This can get dirty inside, and can be cleaned by removing it from the upper intake.
Complaint would be unstable idle, stalling at idle, idle not changing when cold or warm.
EGR modulator/solenoid, computer opens and closes this valve to send vacuum to EGR Valve to open it or close it.