maybe..unlikely..
A bad modulator will make a vacuum leak,but usually they suck tranny fluid into the motor when the diaphram fails,not air,and that will eventually make the tranny run low enough on fluid to afftect shifting,or harm it!..
A vacuum leak else where might kill the motor at idle,if its a large leak,like a PCV valve hose that came off ,or the one to the power brake booster--the smaller hoses wont affect it that badly,though they may cause a rough idle or stumble on acceleration if they are leaky or dissconnected...
You may have weak spark in some cylinders that causes the motor to skip under a load--its under more strain in "high" range than "Low" range,so that might be why it only misses in high range..weak spark will not fire under heavy loads,it blows out and doesn't fire the fuel air mixture..check the plug wires,plugs,and cap and rotor..I bet some are either damaged, or old and cracked,and let spark jump to ground, rather than fire the plugs...
