I have a 95 tahoe that sat for 6 months and now wont shift into 3rd on its own( if you put it in D) if you shift it manually to first it will run and shift great then 2nd and its fine but then you put it into D and it sounds like its kicking down to first or passing gear

I know this is common but any help will be appreciated

the speedometer does not work so this leads me to believe that it could be vss but when I scanned the vehicle with a diagnostic tool it has a speed signal through the computer
any ideas?