1985 Chevy Blazer Loss power around 20 MPH.

Lack of acceleration,chugging,hesitating,jerking,bogging,misfiring etc. could be caused by one of the following below

Oxygen sensor.
Catalytic converter.
Fuel injectors dirty/sticking.
Mass airflow sensor/Airflow meter.
Throttle position sensor.
Crankshaft position sensor
Knock sensor
Manifold absolute pressure sensor.
EGR Valve
Fuel pressure regulator leaking or defective fuel pump.
False air leakage.
Fuel contamination.
Foul/defective spark plugs.
Open spark plug wires.
Ignition coil/Coil packs defective.
Incorrect ignition timing.
Cap and rotor.

Note:If it doesn't apply disregard it

Thanks for the quick response. Im new to this whole thing but im eager to learn. I will look into what you suggested. What would be first thing to look at? Do you think my transmission is bad? It still drives smooth and there is no knocking noise coming from the transmission. It just doesnt go faster than 20 MPH I had to drive it home on second.gear at 15MPH. I hope its not the transmission.