Add a quart, check fluid level with engine running. If level is correct, and still not shifting right, check the transmission filter. You have to drop the transmission pan to get to this. Its a pain in the ASS to do because usually there are 15 or more 3/8" bolts holding it on.
You need to add only one quart on the 99 Ford Ranger. However, you need to check on the gas levels, transmission fitness, and the clutch. These are some of the things that can slow your car. In addition, you need to take your car to a mechanic for fixing.