What's wrong with my Ford Ranger?

1990 Ford Ranger 2.9L V6 4x4 Automatic. Happened overnight. One day it was running amazing, the next not so much. Symptoms:
* Won't go past 55 mph
* Delayed shifting
* Loud sound as if air is escaping while accelerating
Beyond that the truck still drives great. No smoking, overheating, nothing else at all.

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