Here are the electrical diagrams for rangers
http://www.therangerstation.com/tech_library/EDiagrams/index.htm
If you look at the charging system diagram you will see a jumper wire on the Alternator supplies voltage to the dash board gauge, if this wire gets frayed it can cause problems, look inside the connector, there is often only 1 or two strands left connected.
Also check the alternator connector wire going to the dash board gauge.
If you have a voltmeter you should check the voltage at the battery.
12.7vDC or higher with key off
13.8-14.2vDC with key on engine idling, lights and accessories off
Turn on all lights, fans, and anything else that draws power
Voltage should drop then come back up to at least 13.6vDC, that is the voltage regulator in the alternator responding to more amps being needed, the voltage regulator is on the same circuit that supplies the dash gauge.
Raise engine speed to 1,500 or higher RPM, voltage should go up to a maximum of 14.2vDC.
Generally if you have higher speed engine issues it is a sign that the fuel filter needs to be changed, it is limiting full flow, so idle and low speed is not a problem, only higher speeds.
It can also be a sign that the filter "bag" in the tank is getting clogged up, but that's a long shot.
Another higher RPM issue can be exhaust blockage, older Catalytic converters can start to fall apart inside this can limit exhaust flow at higher RPMs so the back pressure at the heads limits fuel entering the cylinders, so you reach a certain speed and that's it, it won't go any faster.
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