Need a little more info

Year?
Engine, 2.9 or 3.0
Manual or auto

There is a REV limiter in computer that prevents high RPMs when in neutral/park or with clutch pedal in.

If RPMs vary while driving, under load, then check throttle linkage, perhaps EGR Valve issue.

 

3.0 automatic 2001

 

First thing would be to change the fuel filter if you don't know when it was last changed.

At higher RPM/demand you could be loosing full fuel flow, so RPMs would start to bounce
And that could also be a fuel pump issue, 2001 Ranger would use/need 65psi fuel pressure.