1995 Ford Ranger running extremely slow

I have a 1995 4 cylinder/2.3 auto Ford Ranger and for over 2 years the truck has been running extremely slow. I’ve been out of work for quite some time now and I haven’t been able to take it to a shop. I bought the Ranger used in 2005 and it originally had 183,000 miles. Back then the little bugger would fly as I’d constantly find myself going 70-75 mph on 55 mph highways.

Now I’m at almost 300,000 miles and the truck suffers just to go 45-50 mph and that’s with it floored. The check engine light hasn’t even worked in years. I took it to Auto Zone and they said that my connector that plugs into the scanner was broken so now I can’t even check to see if it’s throwing any codes. I last had the spark plugs and wires changed at 200,000 miles, but the truck hasn’t had a “real” tune-up since I’ve owned it. I should also point out that everything on the Ranger is original engine and transmission wise.

The only parts I’ve replaced are the heater control valve, EGR valve and the thermostat. I’ve always done regular maintenance such as fluid changes, filters, belts, etc. I should also point out that the truck runs as smooth as can be and is quiet with no off noises or knocks. Even the A/C is still ice cold and the heater will darn near set you on fire in the winter. I know my description is pretty vague, but is there anything in particular that I should be looking for?