Servo run away typically occurs from two sources: Bad (shorted) drive FET(s) or unplugged/defective encoder. Since you swapped drives and it still happens that leaves the encoder. For instant run away to occur you have to typically lose both encoder channels. If it starts up slow and accelerates to full speed it would indicate one channel is defective. If it instantly goes to full speed you have lost both channels.
The above conditions are for systems using standard TTL encoders and where the servo loop is closed in the motor drive. Not knowing your specific system I can only generalize.
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