1989 dakota stalls in gear?

i have a 1989 dakota with 83k auto 3 spd. overdrive.when i bought it, it needed a trans so i put a used one in. the trans worked fine for a month or 2, but as of yesterday, its having problems. it will start and idle perfect, but as soon as i put it in gear (forward or reverse) it stalls violently. i can get the car in drive without stalling if i rev the motor a little and drop it in gear, and the trans will shift fine while driving. as soon as the truck comes to a stop though it dies. it feels like a manual trans when you dont press the clutch in at a stop(mines an auto though??). whats the deal???