Why won't my truck shift into overdrive? on 1994 Dodge Dakota

my truck will not shift into overdrive and the speed odometor does not work either. i replaced the speed control sensor and checked the gear for it and still it is not working. i was told that there is a speed buffer or such but i have not idea where to find this on my truck

Asked by for the 1994 Dodge Dakota
Were you able to get this resolved? Did you get the diagnostic trouble codes in the transmission checked?