My truck will start with a jump but will shut off. battery checked ok,alternator checked ok.plenty of gas. runs at high idle shutsoff at low idle. whats wrong?

If the battery is ok why do you need to jump anyway? most likely its not ok, the alternator do not provide enough power at idle to supply the engine the amount of power needed to keep it running,below 1200rpm you actually use the battery, in the range of 1400 to 2000rpm the alternator will produce a surplus enough to recharge the battery, but if the battery is old it does not recharge enough to start your truck