A common cause for the lean code and stalling is a collapsed or leaking pcv hose, over time the rubber hoses can get soft and oil soaked and can collpse or detrioate and cause a vacuum leak, with the engine running you can listen for a hissing sound and see if you can locate a leak, look closely at the pcv hose ends. This picture is the pcv hose at the vacuum source.The code P1729 is for a 4x4 switch malfunction, double check the code, this probably doesn't relate to your van.

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It won't make it last longer, it prevents the trans from shifting into overdrive, this is helpful when driving on a hilly road or towing because the trans will upshift and downshift alot causing the fluid to heat up.

This code is a generic slip code for the trans, the computer monitors the rpm's at every shift point and also when the torque convertor clutch locks, if this is not accompanied by another code, it will be difficult to determine what caused it. It won't cause the stalling you are experiencing, if the trans fluid hasn't been changed in a while this may help.

I agree, if you are going to spend that much on the torque converter, you would be better off spending the money and have the rest of the trans rebuilt, chances are something else could go wrong soon if you were to replace just the converter. The torque convertor can cause this in and out shift feeling. And I would inspect the pcv and vacuum hoses for the lean code and stalling.