If your vehicle is showing engine stalls when shifted into gear, here are the most likely causes and the parts you'll need to fix each one.
The IAC valve or electronic throttle body adjusts airflow to maintain engine idle when a mechanical load (like shifting into gear) is applied. If it is carbon-fouled or failing, the engine cannot bypass enough air to compensate for the transmission's load and stalls.
A leak in the intake system allows unmetered air into the engine, creating a lean air-fuel mixture. This weakens the engine's idle, causing it to stall when the extra load of the transmission is applied. Often caused by disconnected hoses after recent engine work.
The torque converter clutch locks the engine to the transmission for highway cruising. If the solenoid sticks 'on' at idle, it acts exactly like dumping the clutch on a manual transmission without giving it gas, instantly stalling the engine.
The MAF sensor measures air entering the engine. If it is dirty or failing, the engine computer miscalculates the required fuel, resulting in a weak idle that cannot support the transmission engaging.
Worn spark plugs or weak ignition coils can cause a weak combustion event. The engine might barely run in Park, but the added mechanical resistance of shifting into gear overwhelms it.
Low fluid or a transmission filter that has fallen out of the valve body (often after a recent service) can cause erratic line pressure. This can inadvertently apply the lock-up clutch and stall the engine.