Why would a 1983 Mercury Zephyr idle normally in park but sputter when you put it in gear?

Sounds like the timing could be a little out of whack, when in park there is no load and the base idle is good, but when under load there is a vacuum advance issue and the timing is just out enough to cause the sputtering. Check the cap and rotor for any signs of cross firing or arcing inside the cap, hooped rotors will also show this arcing as witness marks on the rotor.