1995 Chevy Silverado dangerously poor acceleration

The truck starts and idles fine. When I accelerate it starts missing. When I back off the accelaerator it will begin to accelerate again although at a much more gradual rate. Beyond a slight amount of accelerator, it will start to miss. I change the fuel filter on a fairly routine basis and it is equipped with a K&N air filter. The plugs and wires are relatively new. I recently changed the speed sensor (for the second time in the life of the truck) due to malfunction and it had transaxle fluid on it. Is this normal? Could it be related? (It doesn't act the same as a speed sensor malfunction.) Thank you for your advice on this issue.