Vehicles with dual zone heating systems have two blend doors - one for each zone. These are different than the mode doors, which direct the airflow to floor, dash, defrost, etc. Apparently the blend door for the driver's side is working, but not the one for the passenger's side.
Is it the diesel or the transmission doing this shuddering. Could be the transmission, meaning clutch wear and slippage. If in fact it IS the diesel, you could most likely have a dead or plugged injector on one of the cylinders.
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