First question is have you had it scanned? Then do you have a 4L80E or a 4L60E/65E transmission? 4L80E first, if your getting inaccurate shift points check output speed sensor. Internal; the pressure switch manifold could have loose connector pinched wire or no signal to PCM. Won't go into drive; check the main line presure; Drive,Park, or Neutral = 35-171 PSI---Reverse = 67-324 PSI could be an electronic problem, then possible the forward clutch pack could be burnt.
4L60E/65E; inaccurate shift points-possible stuck presure regulator valve, bad Throttle Position Sensor, Vehicle Speed Sensor. Internal
possible burnt forward clutch pack, electronic problem. NEED TO FIRST SCAN COMPUTOR. Then need more detailed info. Just some input, Hope this helps.