95 tahoe not shifting properly

I have a 95 chevy tahoe and recently had some work done on to get it running again after it died on me, but for some reason when I got it going the transmission stopped working right. It has a hard time shifting into 3rd a nd then it wont shift into 4th, could this possibly be a vacuum hose or something similiar and if so where could I find it because im lost

The shifting is done by the pcm from various inputs from specified sensors, make sure the main plug on the trans is securely in place, for starters.