The actual shifting is done by "servos" ... which are hydraulic piston gizmos that take a small hydro signal and amplify it into a larger mechanical force. These servos have lots of 0-rings that can fail .... I had one failed due to a broken cir-clip. May not be the problem ... but a cheap fix if it is.
Both servos are on the backside of the tranny ... one is easy to get to .... one not so easly .... things have to be moved and the tranny rear lowered a bit. Not rocket science ... but there are some pre-loaded springs/screws that can be trouble. A little reading in a shop manual is a good start.