Ford Ranger 2001 High rpm while driving

My son has a 2001 ford ranger 3.0L. It has high rpm when going down the road (almost like low range). The transfer case is in 2H. Shifting is eratic and doesnt seem to have much power. There are no trouble codes. Vacuum is steady but low 14 - 15. Someone suggested the catalytic converter but that checks ok also. Changed tranny fluid and filter. Old stuff was clean and didnt smell. Any help would be great.

What RPMS at what speed? Some more info is needed.

over 2000 at 20 mph and over 3000 at 60 mph (3rd gear) when it finally shifts into overdrive its about 2400 rpm.