The transmission does not care about the diff swap. Try filter and fluid first then check the gov. The shift are not electronically controlled. The only thing to check easy is that the governor gear is not broken. To check the gov. remove the side housing cover on rear driver's side of trans. Push in, remove snap ring and pull the cover off and then see if the gov. can be moved independent of the drive shaft. If so remove the rear cover off the transmission and repl. the gear if broken or the some gov. have a plastic gear that breaks and is repl.