gmc z71 transmission shifting hard

I have a 1995 GMC k1500 z71 w/ 165000 miles on it I have done a full tune up on it. It just recently started to act up, what it started to do was when I started the truck in the morning the check engine light would come on and it would shift very hard like a motor mount was broken. I would turn it off and let it set for a second and it would stop the problem. I has progressively got worse. Now it does it almost everytime I start the truck and when it doesn't initially do it does after a few minutes of driving. I took it to my mechanic he said it was a computer problem so I replaced the computer w/ no luck. He put it on the computer to see if it said anything but it didn't say what it was only that it was a tranny problem. He believes it is a sensor but he is not sure which one now he says he will have to take the tranny apart and test each sensor to find the problem. He is a honest mechanic and a very good friend so he is not lying. If anyone could poss give me some insight as to what it might be that would be awesome. Thanks in advance and GOD BLESS