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There are several reasons that shudder can occur when the torque converter clutch (TCC) engages, expecially at lower speeds. It would be a good idea to check the spark plugs, plug wires, and coil packs first to verify they are intact.
Removing the catalytic converter should not cause any operational problems, but if there is a second oxygen sensor in the exhaust pipe following the converter, the ECM may recognize that the converter is not functioning and set an error code. If there is no sensor downstream of the converter, the ECM should not react to the converter being removed.