Since you stated that you diconnected the driveshaft from the rear diff the play is now only in the driveshaft u-joint or in the tranny. It is normal for the tranny to make that noise while in gear. You are just seeing the backlash or play in the gear that is selected. The only way to know if it is within spec is to look in the tranny service manual for backlash in each gear. I cant speak on familiarity for the z but I think that is normal.
You may want to check the u-joints in the driveshaft and make sure those don't have much play. I am guessing they are probably good too.
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