Chevy van trans won't shift

I have a 95 chevy G30 van with a 5.7 V8. I bought it used about 2 years ago and it has been running and shifting fine until about 6 months ago. I had parked it in the drivway and was gone for a month. When I went to drive it again it would not shift out of first gear. At the same time I noticed the speedometer wasn’t working correctly, and as I hit the brakes the abs light came on and the brakes chatered along with the pulsating pedal. Obviously these are interelated. After two garages and one transmittion shop failed to know what the problem was I turned to researching this on the web. I changed the speed censors on the transmittion and that did nothing. I then found a article that said it could be the ignition switch because the voltage for the trans goes through this switch. So I changed it and after a few days all the problems of the brakes, speedo and transittion stopped. Every once in a while the brkaes would still chatter for a few seconds but everything else seemed OK. Now, today, after putting in a new battery this morning all these problems are back. Now I don’t believe it’s the ignition switch. Does anyone have any idea what the problem REALLY is? I need help. thanks.

Brian