The blazer won't shift out of first unless you let your foot off the gas and push again. We have changed the two shift solenoids and the piston on the one two shift but the problem still exists any suggestions our last idea is the vacuum modulator but we don't know were it is

 

can you manually shift it out of first

 

Yes I can manually shif out of first and if I'm accelerating If I let my foot off the gas it will shift right away