I have a 99 2500 4x4. it started changing gears while driving. from second to thied, second to third. check the batteries and one of the was no good. put a new battery on and now the truck starts off in second gear and wont go into overdrive. no codes yet. thanks in advance

 

Check the alternator output and AC noise levels. There is situations where voltage/amperage issues and/or alternator problems cause the auto tranny to freak out.

 

Mine started some of the same kind of junk and after a lot of checking found out it was a broken wire in the wiring harness going to the transmission. Fixed that and no more problems. LOL