Yeah I don't recall the Ranger being a model known for lasting 20 years.
Kill it. Kill it with fire.
Mine is pretty close to it.
Unless there is a problem in the fuel line, like a grungy line filter, it is almost certainly the emissions system. A completely used-up catalytic converter will cause this, as the emission system will get signals from sending units in the exhaust (casued by oil cooking in the converter) the shut a valve in the intake air, and try to rebreathe the exhaust. The engine will appear to idle normally, but be unable to develop any power or accelerate.
Teriyaki can easily troubleshoot this by the non-environmentally-friendly expedient of detaching the intake air hose at the carb, then driving it up and down the street. If the vehicle accelerates and can get out of first gear with the entire air cleaning/emissions package out of the loop, it is an air problem (and probably caused by emission control crap) and not a carbeuration or fuel issue.