I have a 95 S-10 Blazer, 4X4, 4.3L V6, automatic w/OD, 170000 miles. The problem is that the transmission will not shift into 3RD or 4TH gear. Sometimes it will shift fine and then when I stop or slow down to where it has to shift back to 2ND it will not shift back into third. Sometimes it will shift if I build up enough speed going down a hill and let it drift without giving it any gas for a good distance, and than sometimes not. Once it shifts into 3RD it always shifts into 4TH just fine. There is no CEL on. I had a shop road test it with a computer and they said the trucks computer is calling for 3RD and 4TH gear but it was not shifting. I was told it could be a solenoid not activating or just hanging up. Could this be the problem? If so, where could I find a diagram of the solenoids, which is which, and how do I go about testing them? I was planning to change the filter and fluid this spring any way. No reason I couldn’t do it now while it’s only 40 degrees instead of 80. The cold air is good for the lungs, just not the back. Thanks for all your help.