How do I disable the automatic downshifting in a 1991 Chevy Blazer?

Sounds like the kick down cable is out of adjustment. You can look at the cable coming from the transmission to the throttle body,it has a large end on it that locks into the bracket at the throttle body. The large end has a on that you can push(hold it in) and pull the cable housing toward the fire wall. Release the on,then to adjust,grab the throttle arm with your hand and turn it to wide open throttle,this will pull the cable housing back to its proper adjustment. It will click loud,so don't think you broke it. Good Luck!