Cuts off when I put it in gear

If it only cuts off when you put it in gear then that is something in the transmission that is more than likely engaging the torque converter that stalls the engine. The alternator and or battery would effect the overall ability to start but not necessarily the ability to keep running.

I would suggest starting with scanning for codes to see if there are codes. If you don't have the ability to scan for codes, most parts stores will do it for free. Let me know if we have codes and we can go from there. Thanks