1999 Chevy S-10 Transmission doesn't start in first gear

I had my transmission rebuilt less than 3 years ago, and suddenly my check engine light came on, and now the truck starts off in 3rd gear and overdrive or "D" is weaker than it used to be. When the check engine light goes off, once in a while, the truck is back to normal. That went on for a while, but now it starts off in third all the time. I have to manually shift it everyday now. I still however, don't have first gear, even when I manually shift it. I'm afraid that if I continue with this manually shifting, I will burn out my transmission. Is there something I can do myselft to troubleshoot this problem? All transmission shops want $100 just to diagnos the problem, and we all know there going to tell me that I need a new transmission. They are all the same.