Definitely sounds like the alternator. Possibly when it was tested first time it was putting to spec. Have read similar thing here before where it performs at some RPM's and not other due to worn brushes and defective voltage regulators. From what can tell on Ford Parts website (Part 10346) it seems it's a 95 amp unit. Make sure you get a good replacement with a decent warranty. If you replace it yourself disconnect battery terminals to avoid shorting anything out in the changeover.

Buy 1999 Ford Ranger Parts | FordParts.com