Your transmission range sensor may be going out . My 94 did this and that was the problem. This sensor is on the side of the tranny behind the shift lever. If you replace it buy it from a ford dealership. The part is actually cheaper at the dealership. You will also have to change the plug. The plug is supplied with the new sensor.
Changing this sensor is not that bad. Changing the plug is a little complicated. When you change the plug you have to take each wire out of the old plug and put them in the new one. You have to be carefull not to put them in the plug bakwards(I did). Other than that it is not too bad. The part only costs $50.
The range sensor is behind the shift arm on the side of the tranny. This sensor tells the computer what gear is the tranny is in . If the sensor is going bad the computer will shift the trans in some messed up ways.
when you crawl under the truck from the drivers side. You will see where the cable connects to the shift arm. Directly behind it is the range sensor(it moves with the shift arm). Remove the nut and pull the shift arm off. Then remove the two screws that hold the sensor on . Pull the sensor off and unplug it. Make sure that you disconnect the battery befor you unplug the sensor. If you have Yahoo Instant Messenger , look me up my user name is analreject. I'll help the best I can.