This isn't quite clear. Are you saying it will idle when you drop it in gear and stalls when you give it some throttle or are you saying as soon as you drop it in gear it stalls?
If it idles in gear and stalls when you give it throttle it's an engine problem like TB said (and yes they can rev ok in park/neutral but not in gear cause it takes a lot more fuel to pull the car in gear).
If it stalls as soon as you drop it in gear it's a converter problem (no matter what your shop told you)