This is an automatic?
Could be an issue with the TPS(throttle position sensor) or the throttle cable.
Rangers do have a known issue with the throttle cable stretching, so you never get full throttle but also don't "go" right when you put the gas pedal down.
Above the gas pedal you will see the linkage with the cable hooked up.
Pull up on the gas pedal and see if it moves up, so cable is now sticking out a bit, it shouldn't, if it does people often use "zap straps", A.K.A. wire ties, around the cable to take up the slack.
This is a good fix even if it wasn't the issue
The TPS tells the computer when your foot is pressing on the gas pedal and how much.
The TPS is located on the upper intake and connected to the throttle plate on the opposite side of where the throttle cable is hooked up.
The TPS operates the same as a volume control or light dimmer, it is a variable resistor.
So can be tested with an OHM meter or volt meter.
As you open the throttle the OHMs or Voltage should go up or down, doesn't matter which way.
What maters is that it changes instantly and there are no "dead spots" or sudden jumps, should be nice an steady as you open and close the throttle plate.
Good read here about this sensor:
http://oldfuelinjection.com/index.php?p=30
And make sure and clean the MAF sensor, it could be your problem, but needs to be cleaned now and then in any case.