2003 Trailblazer not starting and gauges sporatic

I have a 2003 Trailblazer with the vortec 4200 in it. Last night my wife cranked it and while I was loading my son it stopped running. It would then turn over, but never crank at times and then sometimes it would not turn over at all. Today I went to Oreilleys and had the battery checked and they said it was fine. I put it back in and am still having the same issue. I can give the key a half turn lights, radio, horn, windows all work fine, but the gauges are jumping all over the place. One minute it will turn the engine over but never crank. The next minute you turn the key and it does not even try to turn over. I had a master certified mechanic look at it real quick before he went to his mothers for the day( Mother's day). He checked some of the fuses and there is a row of 10amps under the hood that was only getting 4v reading and he said they should have been getting 12? Also I can not get the trailblazer out of park. Should I be able to with the key on a half turn? I need help. I was going to pull it, but cant get it out of park. Any help is greatly appreciated.