I need to know what to do to fix the automatic transmission in my 2000 dodge dakota 4x4

Driving down the road, down shifted out of overdrive. The transmission does not down shift to low, has to be manually pulled into second and only works in second and third . Carried to Dodge dealer in Dallas, Tx. After $100 they told me exactly what I already knew, I have second and third and will not down shift into low or into overdrive. They said transmission was bad and needed rebuilt so I put another transmission with 75,000 miles on it, same problems, no change. Mechanic thought that second transmission was bad so he replaced that transmission with third one with 45,000 miles along with the computer on it and still same problem. So I brought it home and no one knows what to do to fix this problem. It seems strange that 3 different transmissions have same problems. Can this be an external problem? I have been told that the transmission's computer needs flashed by several people and then others say it should not be necessary. Could this be the problem that the computer needs flashed or could it be another external problem?