1994 chevy fullsize blazer-hard shifting when hot

i have a 1994 fullsize chevy blazer with the 5.7l 350, auto trans, 4x4. it runs great when started cold and when you let it cool down after being shutoff. but when you start it after you just got done going somewhere and it was at running temp it will want to shift hard like a shift kit and it will not go into overdrive. could it be a temp. sensor in the tranny?